""" Handle the email *forward* and *reply*. phase. There are 3 actors: - contact: who sends emails to alias@sl.co address - SL email handler (this script) - user personal email: to be protected. Should never leak to contact. This script makes sure that in the forward phase, the email that is forwarded to user personal email has the following envelope and header fields: Envelope: mail from: @contact rcpt to: @personal_email Header: From: @contact To: alias@sl.co # so user knows this email is sent to alias Reply-to: special@sl.co # magic HERE And in the reply phase: Envelope: mail from: @contact rcpt to: @contact Header: From: alias@sl.co # so for contact the email comes from alias. magic HERE To: @contact The special@sl.co allows to hide user personal email when user clicks "Reply" to the forwarded email. It should contain the following info: - alias - @contact """ import email import time import uuid from email import encoders from email.message import Message from email.mime.application import MIMEApplication from email.mime.multipart import MIMEMultipart from email.utils import parseaddr, formataddr from io import BytesIO from smtplib import SMTP from aiosmtpd.controller import Controller from aiosmtpd.smtp import Envelope from app import pgp_utils, s3 from app.alias_utils import try_auto_create from app.config import ( EMAIL_DOMAIN, POSTFIX_SERVER, URL, ALIAS_DOMAINS, POSTFIX_SUBMISSION_TLS, UNSUBSCRIBER, LOAD_PGP_EMAIL_HANDLER, ) from app.email_utils import ( send_email, add_dkim_signature, add_or_replace_header, delete_header, email_belongs_to_alias_domains, render, get_orig_message_from_bounce, delete_all_headers_except, get_addrs_from_header, get_spam_info, get_orig_message_from_spamassassin_report, parseaddr_unicode, ) from app.extensions import db from app.greylisting import greylisting_needed from app.log import LOG from app.models import ( Alias, Contact, EmailLog, CustomDomain, User, RefusedEmail, ) from app.utils import random_string from init_app import load_pgp_public_keys from server import create_app # used when an alias receives email from its own mailbox # can happen when user "Reply All" on some email clients _SELF_FORWARDING_STATUS = "550 SL self-forward" # fix the database connection leak issue # use this method instead of create_app def new_app(): app = create_app() @app.teardown_appcontext def shutdown_session(response_or_exc): # same as shutdown_session() in flask-sqlalchemy but this is not enough db.session.remove() # dispose the engine too db.engine.dispose() return app def get_or_create_contact(contact_from_header: str, alias: Alias) -> Contact: """ contact_from_header is the RFC 2047 format FROM header """ # force convert header to string, sometimes contact_from_header is Header object contact_from_header = str(contact_from_header) contact_name, contact_email = parseaddr_unicode(contact_from_header) contact = Contact.get_by(alias_id=alias.id, website_email=contact_email) if contact: if contact.name != contact_name: LOG.d( "Update contact %s name %s to %s", contact, contact.name, contact_name, ) contact.name = contact_name db.session.commit() else: LOG.debug( "create contact for alias %s and contact %s", alias, contact_from_header, ) reply_email = generate_reply_email() contact = Contact.create( user_id=alias.user_id, alias_id=alias.id, website_email=contact_email, name=contact_name, reply_email=reply_email, ) db.session.commit() return contact def replace_header_when_forward(msg: Message, alias: Alias, header: str): """ Replace CC or To header by Reply emails in forward phase """ addrs = get_addrs_from_header(msg, header) # Nothing to do if not addrs: return new_addrs: [str] = [] need_replace = False for addr in addrs: contact_name, contact_email = parseaddr_unicode(addr) # no transformation when alias is already in the header if contact_email == alias.email: new_addrs.append(addr) continue contact = Contact.get_by(alias_id=alias.id, website_email=contact_email) if contact: # update the contact name if needed if contact.name != contact_name: LOG.d( "Update contact %s name %s to %s", contact, contact.name, contact_name, ) contact.name = contact_name db.session.commit() else: LOG.debug( "create contact for alias %s and email %s, header %s", alias, contact_email, header, ) reply_email = generate_reply_email() contact = Contact.create( user_id=alias.user_id, alias_id=alias.id, website_email=contact_email, name=contact_name, reply_email=reply_email, is_cc=header.lower() == "cc", ) db.session.commit() new_addrs.append(contact.new_addr()) need_replace = True if need_replace: new_header = ",".join(new_addrs) LOG.d("Replace %s header, old: %s, new: %s", header, msg[header], new_header) add_or_replace_header(msg, header, new_header) else: LOG.d("No need to replace %s header", header) def replace_header_when_reply(msg: Message, alias: Alias, header: str): """ Replace CC or To Reply emails by original emails """ addrs = get_addrs_from_header(msg, header) # Nothing to do if not addrs: return new_addrs: [str] = [] for addr in addrs: name, reply_email = parseaddr(addr) # no transformation when alias is already in the header if reply_email == alias.email: continue contact = Contact.get_by(reply_email=reply_email) if not contact: LOG.warning( "%s email in reply phase %s must be reply emails", header, reply_email ) # still keep this email in header new_addrs.append(addr) else: new_addrs.append(formataddr((contact.name, contact.website_email))) new_header = ",".join(new_addrs) LOG.d("Replace %s header, old: %s, new: %s", header, msg[header], new_header) add_or_replace_header(msg, header, new_header) def generate_reply_email(): # generate a reply_email, make sure it is unique # not use while loop to avoid infinite loop reply_email = f"reply+{random_string(30)}@{EMAIL_DOMAIN}" for _ in range(1000): if not Contact.get_by(reply_email=reply_email): # found! break reply_email = f"reply+{random_string(30)}@{EMAIL_DOMAIN}" return reply_email def should_append_alias(msg: Message, address: str): """whether an alias should be appended to TO header in message""" if msg["To"] and address in msg["To"]: return False if msg["Cc"] and address in msg["Cc"]: return False return True _MIME_HEADERS = [ "MIME-Version", "Content-Type", "Content-Disposition", "Content-Transfer-Encoding", ] _MIME_HEADERS = [h.lower() for h in _MIME_HEADERS] def prepare_pgp_message(orig_msg: Message, pgp_fingerprint: str): msg = MIMEMultipart("encrypted", protocol="application/pgp-encrypted") # copy all headers from original message except all standard MIME headers for i in reversed(range(len(orig_msg._headers))): header_name = orig_msg._headers[i][0].lower() if header_name.lower() not in _MIME_HEADERS: msg[header_name] = orig_msg._headers[i][1] # Delete unnecessary headers in orig_msg except to save space delete_all_headers_except( orig_msg, _MIME_HEADERS, ) first = MIMEApplication( _subtype="pgp-encrypted", _encoder=encoders.encode_7or8bit, _data="" ) first.set_payload("Version: 1") msg.attach(first) second = MIMEApplication("octet-stream", _encoder=encoders.encode_7or8bit) second.add_header("Content-Disposition", "inline") # encrypt original message encrypted_data = pgp_utils.encrypt_file( BytesIO(orig_msg.as_bytes()), pgp_fingerprint ) second.set_payload(encrypted_data) msg.attach(second) return msg def handle_forward(envelope, smtp: SMTP, msg: Message, rcpt_to: str) -> (bool, str): """return whether an email has been delivered and the smtp status ("250 Message accepted", "550 Non-existent email address", etc) """ address = rcpt_to.lower() # alias@SL alias = Alias.get_by(email=address) if not alias: LOG.d("alias %s not exist. Try to see if it can be created on the fly", address) alias = try_auto_create(address) if not alias: LOG.d("alias %s cannot be created on-the-fly, return 550", address) return False, "550 SL E3" mailbox = alias.mailbox mailbox_email = mailbox.email user = alias.user # Sometimes when user clicks on "reply all" # an email is sent to the same alias that the previous message is destined to if envelope.mail_from == mailbox_email: # nothing to do LOG.d("Forward from %s to %s, nothing to do", envelope.mail_from, mailbox_email) return False, _SELF_FORWARDING_STATUS contact = get_or_create_contact(msg["From"], alias) spam_check = True # create PGP email if needed if mailbox.pgp_finger_print and user.is_premium(): LOG.d("Encrypt message using mailbox %s", mailbox) msg = prepare_pgp_message(msg, mailbox.pgp_finger_print) # no need to spam check for encrypted message spam_check = False if spam_check: is_spam, spam_status = get_spam_info(msg) if is_spam: LOG.warning("Email detected as spam. Alias: %s, from: %s", alias, contact) handle_spam(contact, alias, msg, user, mailbox_email, spam_status) return False, "550 SL E1" forward_log = EmailLog.create(contact_id=contact.id, user_id=contact.user_id) if alias.enabled: # add custom header add_or_replace_header(msg, "X-SimpleLogin-Type", "Forward") # remove reply-to & sender header if present delete_header(msg, "Reply-To") delete_header(msg, "Sender") # change the from header so the sender comes from @SL # so it can pass DMARC check # replace the email part in from: header contact_from_header = msg["From"] new_from_header = contact.new_addr() add_or_replace_header(msg, "From", new_from_header) LOG.d("new_from_header:%s, old header %s", new_from_header, contact_from_header) # replace CC & To emails by reply-email for all emails that are not alias replace_header_when_forward(msg, alias, "Cc") replace_header_when_forward(msg, alias, "To") # append alias into the TO header if it's not present in To or CC if should_append_alias(msg, alias.email): LOG.d("append alias %s to TO header %s", alias, msg["To"]) if msg["To"]: to_header = msg["To"] + "," + alias.email else: to_header = alias.email add_or_replace_header(msg, "To", to_header.strip()) # add List-Unsubscribe header if UNSUBSCRIBER: unsubscribe_link = f"mailto:{UNSUBSCRIBER}?subject={alias.id}=" add_or_replace_header(msg, "List-Unsubscribe", f"<{unsubscribe_link}>") else: unsubscribe_link = f"{URL}/dashboard/unsubscribe/{alias.id}" add_or_replace_header(msg, "List-Unsubscribe", f"<{unsubscribe_link}>") add_or_replace_header( msg, "List-Unsubscribe-Post", "List-Unsubscribe=One-Click" ) add_dkim_signature(msg, EMAIL_DOMAIN) LOG.d( "Forward mail from %s to %s, mail_options %s, rcpt_options %s ", contact.website_email, mailbox_email, envelope.mail_options, envelope.rcpt_options, ) # smtp.send_message has UnicodeEncodeErroremail issue # encode message raw directly instead smtp.sendmail( contact.reply_email, mailbox_email, msg.as_bytes(), envelope.mail_options, envelope.rcpt_options, ) else: LOG.d("%s is disabled, do not forward", alias) forward_log.blocked = True db.session.commit() return True, "250 Message accepted for delivery" def handle_reply(envelope, smtp: SMTP, msg: Message, rcpt_to: str) -> (bool, str): """ return whether an email has been delivered and the smtp status ("250 Message accepted", "550 Non-existent email address", etc) """ reply_email = rcpt_to.lower() # reply_email must end with EMAIL_DOMAIN if not reply_email.endswith(EMAIL_DOMAIN): LOG.warning(f"Reply email {reply_email} has wrong domain") return False, "550 SL E2" contact = Contact.get_by(reply_email=reply_email) if not contact: LOG.warning(f"No such forward-email with {reply_email} as reply-email") return False, "550 SL E4" alias = contact.alias address: str = contact.alias.email alias_domain = address[address.find("@") + 1 :] # alias must end with one of the ALIAS_DOMAINS or custom-domain if not email_belongs_to_alias_domains(alias.email): if not CustomDomain.get_by(domain=alias_domain): return False, "550 SL E5" user = alias.user mailbox_email = alias.mailbox_email() # bounce email initiated by Postfix # can happen in case emails cannot be delivered to user-email # in this case Postfix will try to send a bounce report to original sender, which is # the "reply email" if envelope.mail_from == "<>": LOG.warning( "Bounce when sending to alias %s from %s, user %s", alias, contact.website_email, alias.user, ) handle_bounce(contact, alias, msg, user, mailbox_email) return False, "550 SL E6" # only mailbox can send email to the reply-email if envelope.mail_from.lower() != mailbox_email.lower(): LOG.warning( f"Reply email can only be used by mailbox. " f"Actual mail_from: %s. msg from header: %s, Mailbox %s. reply_email %s", envelope.mail_from, msg["From"], mailbox_email, reply_email, ) send_email( mailbox_email, f"Reply from your alias {alias.email} only works from your mailbox", render( "transactional/reply-must-use-personal-email.txt", name=user.name, alias=alias.email, sender=envelope.mail_from, mailbox_email=mailbox_email, ), render( "transactional/reply-must-use-personal-email.html", name=user.name, alias=alias.email, sender=envelope.mail_from, mailbox_email=mailbox_email, ), ) # Notify sender that they cannot send emails to this address send_email( envelope.mail_from, f"Your email ({envelope.mail_from}) is not allowed to send emails to {reply_email}", render( "transactional/send-from-alias-from-unknown-sender.txt", sender=envelope.mail_from, reply_email=reply_email, ), render( "transactional/send-from-alias-from-unknown-sender.html", sender=envelope.mail_from, reply_email=reply_email, ), ) return False, "550 SL E7" delete_header(msg, "DKIM-Signature") delete_header(msg, "Received") # make the email comes from alias add_or_replace_header(msg, "From", alias.email) # some email providers like ProtonMail adds automatically the Reply-To field # make sure to delete it delete_header(msg, "Reply-To") # remove sender header if present as this could reveal user real email delete_header(msg, "Sender") delete_header(msg, "X-Sender") replace_header_when_reply(msg, alias, "To") replace_header_when_reply(msg, alias, "Cc") # Received-SPF is injected by postfix-policyd-spf-python can reveal user original email delete_header(msg, "Received-SPF") LOG.d( "send email from %s to %s, mail_options:%s,rcpt_options:%s", alias.email, contact.website_email, envelope.mail_options, envelope.rcpt_options, ) if alias_domain in ALIAS_DOMAINS: add_dkim_signature(msg, alias_domain) # add DKIM-Signature for custom-domain alias else: custom_domain: CustomDomain = CustomDomain.get_by(domain=alias_domain) if custom_domain.dkim_verified: add_dkim_signature(msg, alias_domain) smtp.sendmail( alias.email, contact.website_email, msg.as_bytes(), envelope.mail_options, envelope.rcpt_options, ) EmailLog.create(contact_id=contact.id, is_reply=True, user_id=contact.user_id) db.session.commit() return True, "250 Message accepted for delivery" def handle_bounce( contact: Contact, alias: Alias, msg: Message, user: User, mailbox_email: str ): address = alias.email email_log: EmailLog = EmailLog.create( contact_id=contact.id, bounced=True, user_id=contact.user_id ) db.session.commit() nb_bounced = EmailLog.filter_by(contact_id=contact.id, bounced=True).count() disable_alias_link = f"{URL}/dashboard/unsubscribe/{alias.id}" # Store the bounced email orig_msg = get_orig_message_from_bounce(msg) # generate a name for the email random_name = str(uuid.uuid4()) full_report_path = f"refused-emails/full-{random_name}.eml" s3.upload_email_from_bytesio(full_report_path, BytesIO(msg.as_bytes()), random_name) file_path = None if orig_msg: file_path = f"refused-emails/{random_name}.eml" s3.upload_email_from_bytesio( file_path, BytesIO(orig_msg.as_bytes()), random_name ) refused_email = RefusedEmail.create( path=file_path, full_report_path=full_report_path, user_id=user.id ) db.session.flush() email_log.refused_email_id = refused_email.id db.session.commit() LOG.d("Create refused email %s", refused_email) refused_email_url = ( URL + f"/dashboard/refused_email?highlight_id=" + str(email_log.id) ) # inform user if this is the first bounced email if nb_bounced == 1: LOG.d( "Inform user %s about bounced email sent by %s to alias %s", user, contact.website_email, address, ) send_email( # use user mail here as only user is authenticated to see the refused email user.email, f"Email from {contact.website_email} to {address} cannot be delivered to your inbox", render( "transactional/bounced-email.txt", name=user.name, alias=alias, website_email=contact.website_email, disable_alias_link=disable_alias_link, refused_email_url=refused_email_url, mailbox_email=mailbox_email, ), render( "transactional/bounced-email.html", name=user.name, alias=alias, website_email=contact.website_email, disable_alias_link=disable_alias_link, refused_email_url=refused_email_url, mailbox_email=mailbox_email, ), # cannot include bounce email as it can contain spammy text # bounced_email=msg, ) # disable the alias the second time email is bounced elif nb_bounced >= 2: LOG.d( "Bounce happens again with alias %s from %s. Disable alias now ", address, contact.website_email, ) alias.enabled = False db.session.commit() send_email( # use user mail here as only user is authenticated to see the refused email user.email, f"Alias {address} has been disabled due to second undelivered email from {contact.website_email}", render( "transactional/automatic-disable-alias.txt", name=user.name, alias=alias, website_email=contact.website_email, refused_email_url=refused_email_url, mailbox_email=mailbox_email, ), render( "transactional/automatic-disable-alias.html", name=user.name, alias=alias, website_email=contact.website_email, refused_email_url=refused_email_url, mailbox_email=mailbox_email, ), # cannot include bounce email as it can contain spammy text # bounced_email=msg, ) def handle_spam( contact: Contact, alias: Alias, msg: Message, user: User, mailbox_email: str, spam_status: str, ): email_log: EmailLog = EmailLog.create( contact_id=contact.id, user_id=contact.user_id, is_spam=True, spam_status=spam_status, ) db.session.commit() # Store the report & original email orig_msg = get_orig_message_from_spamassassin_report(msg) # generate a name for the email random_name = str(uuid.uuid4()) full_report_path = f"spams/full-{random_name}.eml" s3.upload_email_from_bytesio(full_report_path, BytesIO(msg.as_bytes()), random_name) file_path = None if orig_msg: file_path = f"spams/{random_name}.eml" s3.upload_email_from_bytesio( file_path, BytesIO(orig_msg.as_bytes()), random_name ) refused_email = RefusedEmail.create( path=file_path, full_report_path=full_report_path, user_id=user.id ) db.session.flush() email_log.refused_email_id = refused_email.id db.session.commit() LOG.d("Create spam email %s", refused_email) refused_email_url = ( URL + f"/dashboard/refused_email?highlight_id=" + str(email_log.id) ) disable_alias_link = f"{URL}/dashboard/unsubscribe/{alias.id}" # inform user LOG.d( "Inform user %s about spam email sent by %s to alias %s", user, contact.website_email, alias.email, ) send_email( mailbox_email, f"Email from {contact.website_email} to {alias.email} is detected as spam", render( "transactional/spam-email.txt", name=user.name, alias=alias, website_email=contact.website_email, disable_alias_link=disable_alias_link, refused_email_url=refused_email_url, ), render( "transactional/spam-email.html", name=user.name, alias=alias, website_email=contact.website_email, disable_alias_link=disable_alias_link, refused_email_url=refused_email_url, ), ) def handle_unsubscribe(envelope: Envelope): msg = email.message_from_bytes(envelope.original_content) # format: alias_id: subject = msg["Subject"] try: # subject has the format {alias.id}= if subject.endswith("="): alias_id = int(subject[:-1]) # some email providers might strip off the = suffix else: alias_id = int(subject) alias = Alias.get(alias_id) except Exception: LOG.warning("Cannot parse alias from subject %s", msg["Subject"]) return "550 SL E8" if not alias: LOG.warning("No such alias %s", alias_id) return "550 SL E9" # This sender cannot unsubscribe if alias.mailbox_email() != envelope.mail_from: LOG.d("%s cannot disable alias %s", envelope.mail_from, alias) return "550 SL E10" # Sender is owner of this alias alias.enabled = False db.session.commit() user = alias.user enable_alias_url = URL + f"/dashboard/?highlight_alias_id={alias.id}" send_email( envelope.mail_from, f"Alias {alias.email} has been disabled successfully", render( "transactional/unsubscribe-disable-alias.txt", user=user, alias=alias.email, enable_alias_url=enable_alias_url, ), render( "transactional/unsubscribe-disable-alias.html", user=user, alias=alias.email, enable_alias_url=enable_alias_url, ), ) return "250 Unsubscribe request accepted" def handle(envelope: Envelope, smtp: SMTP) -> str: """Return SMTP status""" # unsubscribe request if UNSUBSCRIBER and envelope.rcpt_tos == [UNSUBSCRIBER]: LOG.d("Handle unsubscribe request from %s", envelope.mail_from) return handle_unsubscribe(envelope) # Whether it's necessary to apply greylisting if greylisting_needed(envelope.mail_from, envelope.rcpt_tos): LOG.warning( "Grey listing applied for %s %s", envelope.mail_from, envelope.rcpt_tos ) return "421 SL Retry later" # result of all deliveries # each element is a couple of whether the delivery is successful and the smtp status res: [(bool, str)] = [] for rcpt_to in envelope.rcpt_tos: msg = email.message_from_bytes(envelope.original_content) # Reply case # recipient starts with "reply+" or "ra+" (ra=reverse-alias) prefix if rcpt_to.startswith("reply+") or rcpt_to.startswith("ra+"): LOG.debug(">>> Reply phase %s -> %s", envelope.mail_from, rcpt_to) is_delivered, smtp_status = handle_reply(envelope, smtp, msg, rcpt_to) res.append((is_delivered, smtp_status)) else: # Forward case LOG.debug(">>> Forward phase %s -> %s", envelope.mail_from, rcpt_to) is_delivered, smtp_status = handle_forward(envelope, smtp, msg, rcpt_to) res.append((is_delivered, smtp_status)) # special handling for self-forwarding # just consider success delivery in this case if len(res) == 1 and res[0][1] == _SELF_FORWARDING_STATUS: LOG.d("Self-forwarding, ignore") return "250 SL OK" for (is_success, smtp_status) in res: # Consider all deliveries successful if 1 delivery is successful if is_success: return smtp_status # Failed delivery for all, return the first failure return res[0][1] class MailHandler: async def handle_DATA(self, server, session, envelope: Envelope): LOG.debug( "===>> New message, mail from %s, rctp tos %s ", envelope.mail_from, envelope.rcpt_tos, ) if POSTFIX_SUBMISSION_TLS: smtp = SMTP(POSTFIX_SERVER, 587) smtp.starttls() else: smtp = SMTP(POSTFIX_SERVER, 25) app = new_app() with app.app_context(): return handle(envelope, smtp) if __name__ == "__main__": controller = Controller(MailHandler(), hostname="0.0.0.0", port=20381) controller.start() LOG.d("Start mail controller %s %s", controller.hostname, controller.port) if LOAD_PGP_EMAIL_HANDLER: LOG.warning("LOAD PGP keys") app = create_app() with app.app_context(): load_pgp_public_keys(app) while True: time.sleep(2)