""" Handle the email *forward* and *reply*. phase. There are 3 actors: - website: who sends emails to alias@sl.co address - SL email handler (this script) - user personal email: to be protected. Should never leak to website. 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: @website rcpt to: @personal_email Header: From: @website 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: @website rcpt to: @website Header: From: alias@sl.co # so for website the email comes from alias. magic HERE To: @website 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 - @website """ import time from email import encoders from email.message import Message from email.mime.application import MIMEApplication from email.mime.multipart import MIMEMultipart from email.parser import Parser from email.policy import SMTPUTF8 from io import BytesIO from smtplib import SMTP from typing import Optional from aiosmtpd.controller import Controller from app import pgp_utils, s3 from app.config import ( EMAIL_DOMAIN, POSTFIX_SERVER, URL, ALIAS_DOMAINS, POSTFIX_SUBMISSION_TLS, ) from app.email_utils import ( get_email_name, get_email_part, send_email, add_dkim_signature, get_email_domain_part, add_or_replace_header, delete_header, send_cannot_create_directory_alias, send_cannot_create_domain_alias, email_belongs_to_alias_domains, render, get_orig_message_from_bounce, delete_all_headers_except, ) from app.extensions import db from app.log import LOG from app.models import ( GenEmail, ForwardEmail, ForwardEmailLog, CustomDomain, Directory, User, DeletedAlias, RefusedEmail, ) from app.utils import random_string from server import create_app # 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 try_auto_create(alias: str) -> Optional[GenEmail]: """Try to auto-create the alias using directory or catch-all domain """ gen_email = try_auto_create_catch_all_domain(alias) if not gen_email: gen_email = try_auto_create_directory(alias) return gen_email def try_auto_create_directory(alias: str) -> Optional[GenEmail]: """ Try to create an alias with directory """ # check if alias belongs to a directory, ie having directory/anything@EMAIL_DOMAIN format if email_belongs_to_alias_domains(alias): # if there's no directory separator in the alias, no way to auto-create it if "/" not in alias and "+" not in alias and "#" not in alias: return None # alias contains one of the 3 special directory separator: "/", "+" or "#" if "/" in alias: sep = "/" elif "+" in alias: sep = "+" else: sep = "#" directory_name = alias[: alias.find(sep)] LOG.d("directory_name %s", directory_name) directory = Directory.get_by(name=directory_name) if not directory: return None dir_user: User = directory.user if not dir_user.can_create_new_alias(): send_cannot_create_directory_alias(dir_user, alias, directory_name) return None # if alias has been deleted before, do not auto-create it if DeletedAlias.get_by(email=alias, user_id=directory.user_id): LOG.warning( "Alias %s was deleted before, cannot auto-create using directory %s, user %s", alias, directory_name, dir_user, ) return None LOG.d("create alias %s for directory %s", alias, directory) gen_email = GenEmail.create( email=alias, user_id=directory.user_id, directory_id=directory.id, mailbox_id=dir_user.default_mailbox_id, ) db.session.commit() return gen_email def try_auto_create_catch_all_domain(alias: str) -> Optional[GenEmail]: """Try to create an alias with catch-all domain""" # try to create alias on-the-fly with custom-domain catch-all feature # check if alias is custom-domain alias and if the custom-domain has catch-all enabled alias_domain = get_email_domain_part(alias) custom_domain = CustomDomain.get_by(domain=alias_domain) if not custom_domain: return None # custom_domain exists if not custom_domain.catch_all: return None # custom_domain has catch-all enabled domain_user: User = custom_domain.user if not domain_user.can_create_new_alias(): send_cannot_create_domain_alias(domain_user, alias, alias_domain) return None # if alias has been deleted before, do not auto-create it if DeletedAlias.get_by(email=alias, user_id=custom_domain.user_id): LOG.warning( "Alias %s was deleted before, cannot auto-create using domain catch-all %s, user %s", alias, custom_domain, domain_user, ) return None LOG.d("create alias %s for domain %s", alias, custom_domain) gen_email = GenEmail.create( email=alias, user_id=custom_domain.user_id, custom_domain_id=custom_domain.id, automatic_creation=True, mailbox_id=domain_user.default_mailbox_id, ) db.session.commit() return gen_email def get_or_create_forward_email( website_from_header: str, gen_email: GenEmail ) -> ForwardEmail: """ website_from_header can be the full-form email, i.e. "First Last " """ website_email = get_email_part(website_from_header) forward_email = ForwardEmail.get_by( gen_email_id=gen_email.id, website_email=website_email ) if forward_email: # update the website_from if needed if forward_email.website_from != website_from_header: LOG.d("Update From header for %s", forward_email) forward_email.website_from = website_from_header db.session.commit() else: LOG.debug( "create forward email for alias %s and website email %s", gen_email, website_from_header, ) # 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 ForwardEmail.get_by(reply_email=reply_email): # found! break reply_email = f"reply+{random_string(30)}@{EMAIL_DOMAIN}" forward_email = ForwardEmail.create( gen_email_id=gen_email.id, website_email=website_email, website_from=website_from_header, reply_email=reply_email, ) db.session.commit() return forward_email def should_append_alias(msg, alias): """whether an alias should be appened to TO header in message""" if msg["To"] and alias in msg["To"]: return False if msg["Cc"] and alias in msg["Cc"]: return False return True def prepare_pgp_message(orig_msg: Message, pgp_fingerprint: str): msg = MIMEMultipart("encrypted", protocol="application/pgp-encrypted") # copy all headers from original message except the "Content-Type" for i in reversed(range(len(orig_msg._headers))): header_name = orig_msg._headers[i][0].lower() if header_name != "Content-Type".lower(): 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-Version", "Content-Type", "Content-Disposition", "Content-Transfer-Encoding", ], ) 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(orig_msg.as_string(), pgp_fingerprint) second.set_payload(encrypted_data) msg.attach(second) return msg def handle_forward(envelope, smtp: SMTP, msg: Message, rcpt_to: str) -> str: """return *status_code message*""" alias = rcpt_to.lower() # alias@SL gen_email = GenEmail.get_by(email=alias) if not gen_email: LOG.d("alias %s not exist. Try to see if it can be created on the fly", alias) gen_email = try_auto_create(alias) if not gen_email: LOG.d("alias %s cannot be created on-the-fly, return 510", alias) return "510 Email not exist" mailbox = gen_email.mailbox mailbox_email = mailbox.email # create PGP email if needed if mailbox.pgp_finger_print: LOG.d("Encrypt message using mailbox %s", mailbox) msg = prepare_pgp_message(msg, mailbox.pgp_finger_print) forward_email = get_or_create_forward_email(msg["From"], gen_email) forward_log = ForwardEmailLog.create(forward_id=forward_email.id) if gen_email.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 website_from_header = msg["From"] website_email = get_email_part(website_from_header) from_header = ( get_email_name(website_from_header) + ("" if get_email_name(website_from_header) == "" else " - ") + website_email.replace("@", " at ") + f" <{forward_email.reply_email}>" ) add_or_replace_header(msg, "From", from_header) LOG.d("new from header:%s", from_header) # append alias into the TO header if it's not present in To or CC if should_append_alias(msg, alias): LOG.d("append alias %s to TO header %s", alias, msg["To"]) if msg["To"]: to_header = msg["To"] + "," + alias else: to_header = alias add_or_replace_header(msg, "To", to_header) # add List-Unsubscribe header unsubscribe_link = f"{URL}/dashboard/unsubscribe/{gen_email.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 ", website_email, mailbox_email, envelope.mail_options, envelope.rcpt_options, ) # smtp.send_message has UnicodeEncodeErroremail issue # encode message raw directly instead msg_raw = msg.as_string().encode() smtp.sendmail( forward_email.reply_email, mailbox_email, msg_raw, envelope.mail_options, envelope.rcpt_options, ) else: LOG.d("%s is disabled, do not forward", gen_email) forward_log.blocked = True db.session.commit() return "250 Message accepted for delivery" def handle_reply(envelope, smtp: SMTP, msg: Message, rcpt_to: str) -> str: 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 "550 wrong reply email" forward_email = ForwardEmail.get_by(reply_email=reply_email) if not forward_email: LOG.warning(f"No such forward-email with {reply_email} as reply-email") return "550 wrong reply email" alias: str = forward_email.gen_email.email alias_domain = alias[alias.find("@") + 1 :] # alias must end with one of the ALIAS_DOMAINS or custom-domain if not email_belongs_to_alias_domains(alias): if not CustomDomain.get_by(domain=alias_domain): return "550 alias unknown by SimpleLogin" gen_email = forward_email.gen_email user = gen_email.user mailbox_email = gen_email.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.error( "Bounce when sending to alias %s from %s, user %s", alias, forward_email.website_from, gen_email.user, ) handle_bounce( alias, envelope, forward_email, gen_email, msg, smtp, user, mailbox_email ) return "550 ignored" # 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 user email. Actual mail_from: %s. msg from header: %s, User email %s. reply_email %s", envelope.mail_from, msg["From"], mailbox_email, reply_email, ) user = gen_email.user send_email( mailbox_email, f"Reply from your alias {alias} only works from your mailbox", render( "transactional/reply-must-use-personal-email.txt", name=user.name, alias=alias, sender=envelope.mail_from, mailbox_email=mailbox_email, ), render( "transactional/reply-must-use-personal-email.html", name=user.name, alias=alias, 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, ), "", ) return "550 ignored" delete_header(msg, "DKIM-Signature") # the email comes from alias add_or_replace_header(msg, "From", alias) # 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") add_or_replace_header(msg, "To", forward_email.website_email) # add List-Unsubscribe header unsubscribe_link = f"{URL}/dashboard/unsubscribe/{forward_email.gen_email_id}" add_or_replace_header(msg, "List-Unsubscribe", f"<{unsubscribe_link}>") add_or_replace_header(msg, "List-Unsubscribe-Post", "List-Unsubscribe=One-Click") # 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, forward_email.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) msg_raw = msg.as_string().encode() smtp.sendmail( alias, forward_email.website_email, msg_raw, envelope.mail_options, envelope.rcpt_options, ) ForwardEmailLog.create(forward_id=forward_email.id, is_reply=True) db.session.commit() return "250 Message accepted for delivery" def handle_bounce( alias, envelope, forward_email, gen_email, msg, smtp, user, mailbox_email ): fel: ForwardEmailLog = ForwardEmailLog.create( forward_id=forward_email.id, bounced=True ) db.session.commit() nb_bounced = ForwardEmailLog.filter_by( forward_id=forward_email.id, bounced=True ).count() disable_alias_link = f"{URL}/dashboard/unsubscribe/{gen_email.id}" # Store the bounced email random_name = random_string(50) full_report_path = f"refused-emails/full-{random_name}.eml" s3.upload_from_bytesio(full_report_path, BytesIO(msg.as_bytes())) file_path = f"refused-emails/{random_name}.eml" orig_msg = get_orig_message_from_bounce(msg) s3.upload_from_bytesio(file_path, BytesIO(orig_msg.as_bytes())) refused_email = RefusedEmail.create( path=file_path, full_report_path=full_report_path, user_id=user.id ) db.session.flush() fel.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_fel_id=" + str(fel.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, forward_email.website_from, alias, ) send_email( mailbox_email, f"Email from {forward_email.website_from} to {alias} cannot be delivered to your inbox", render( "transactional/bounced-email.txt", name=user.name, alias=alias, website_from=forward_email.website_from, website_email=forward_email.website_email, disable_alias_link=disable_alias_link, refused_email_url=refused_email_url, ), render( "transactional/bounced-email.html", name=user.name, alias=alias, website_from=forward_email.website_from, website_email=forward_email.website_email, disable_alias_link=disable_alias_link, refused_email_url=refused_email_url, ), # 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 ", alias, forward_email.website_from, ) gen_email.enabled = False db.session.commit() send_email( mailbox_email, f"Alias {alias} has been disabled due to second undelivered email from {forward_email.website_from}", render( "transactional/automatic-disable-alias.txt", name=user.name, alias=alias, website_from=forward_email.website_from, website_email=forward_email.website_email, ), render( "transactional/automatic-disable-alias.html", name=user.name, alias=alias, website_from=forward_email.website_from, website_email=forward_email.website_email, ), bounced_email=msg, ) class MailHandler: async def handle_DATA(self, server, session, envelope): LOG.debug(">>> New message <<<") LOG.debug("Mail from %s", envelope.mail_from) LOG.debug("Rcpt to %s", envelope.rcpt_tos) message_data = envelope.content.decode("utf8", errors="replace") if POSTFIX_SUBMISSION_TLS: smtp = SMTP(POSTFIX_SERVER, 587) smtp.starttls() else: smtp = SMTP(POSTFIX_SERVER, 25) msg = Parser(policy=SMTPUTF8).parsestr(message_data) for rcpt_to in envelope.rcpt_tos: # 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") app = new_app() with app.app_context(): return handle_reply(envelope, smtp, msg, rcpt_to) else: # Forward case LOG.debug("Forward phase") app = new_app() with app.app_context(): return handle_forward(envelope, smtp, msg, rcpt_to) 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) while True: time.sleep(2)