diff --git a/cron.py b/cron.py
index 0232c342..49f3f19e 100644
--- a/cron.py
+++ b/cron.py
@@ -68,6 +68,7 @@ from app.models import (
     PartnerUser,
     ApiToCookieToken,
 )
+from app.pgp_utils import load_public_key_and_check, PGPException
 from app.proton.utils import get_proton_partner
 from app.utils import sanitize_email
 from server import create_light_app
@@ -741,6 +742,9 @@ def sanity_check():
     LOG.d("check mailbox valid domain")
     check_mailbox_valid_domain()
 
+    LOG.d("check mailbox valid PGP keys")
+    check_mailbox_valid_pgp_keys()
+
     LOG.d(
         """check if there's an email that starts with "\u200f" (right-to-left mark (RLM))"""
     )
@@ -765,7 +769,7 @@ def check_mailbox_valid_domain():
     )
     mailbox_ids = [e[0] for e in mailbox_ids]
     # iterate over id instead of mailbox directly
-    # as a mailbox can be deleted during the sleep time
+    # as a mailbox can be deleted in the meantime
     for mailbox_id in mailbox_ids:
         mailbox = Mailbox.get(mailbox_id)
         # a mailbox has been deleted
@@ -821,6 +825,40 @@ def check_mailbox_valid_domain():
         Session.commit()
 
 
+def check_mailbox_valid_pgp_keys():
+    mailbox_ids = (
+        Session.query(Mailbox.id)
+        .filter(
+            Mailbox.verified.is_(True),
+            Mailbox.pgp_public_key.isnot(None),
+            Mailbox.disable_pgp.is_(False),
+        )
+        .all()
+    )
+    mailbox_ids = [e[0] for e in mailbox_ids]
+    # iterate over id instead of mailbox directly
+    # as a mailbox can be deleted in the meantime
+    for mailbox_id in mailbox_ids:
+        mailbox = Mailbox.get(mailbox_id)
+        # a mailbox has been deleted
+        if not mailbox:
+            continue
+
+        try:
+            load_public_key_and_check(mailbox.pgp_public_key)
+        except PGPException:
+            LOG.i(f"{mailbox} PGP key invalid")
+            send_email(
+                mailbox.user.email,
+                f"Mailbox {mailbox.email}'s PGP Key is invalid",
+                render(
+                    "transactional/invalid-mailbox-pgp-key.txt.jinja2",
+                    mailbox=mailbox,
+                ),
+                retries=3,
+            )
+
+
 def check_custom_domain():
     LOG.d("Check verified domain for DNS issues")
 
diff --git a/email_handler.py b/email_handler.py
index da33caf5..b09f5975 100644
--- a/email_handler.py
+++ b/email_handler.py
@@ -162,7 +162,12 @@ from app.models import (
     Notification,
     VerpType,
 )
-from app.pgp_utils import PGPException, sign_data_with_pgpy, sign_data
+from app.pgp_utils import (
+    PGPException,
+    sign_data_with_pgpy,
+    sign_data,
+    load_public_key_and_check,
+)
 from app.utils import sanitize_email
 from init_app import load_pgp_public_keys
 from server import create_light_app
@@ -502,7 +507,12 @@ def prepare_pgp_message(
         encrypted_data = pgp_utils.encrypt_file(BytesIO(msg_bytes), pgp_fingerprint)
         second.set_payload(encrypted_data)
     except PGPException:
-        LOG.w("Cannot encrypt using python-gnupg, use pgpy")
+        LOG.w(
+            "Cannot encrypt using python-gnupg, check if public key is valid and try with pgpy"
+        )
+        # check if the public key is valid
+        load_public_key_and_check(public_key)
+
         encrypted = pgp_utils.encrypt_file_with_pgpy(msg_bytes, public_key)
         second.set_payload(str(encrypted))
 
diff --git a/templates/emails/transactional/invalid-mailbox-pgp-key.txt.jinja2 b/templates/emails/transactional/invalid-mailbox-pgp-key.txt.jinja2
new file mode 100644
index 00000000..4a001d6f
--- /dev/null
+++ b/templates/emails/transactional/invalid-mailbox-pgp-key.txt.jinja2
@@ -0,0 +1,11 @@
+{% extends "base.txt.jinja2" %}
+
+{% block content %}
+
+  We have detected that your mailbox {{ mailbox.email }}'s PGP key is invalid.
+
+  A potential cause is the key is already expired.
+
+  Please update the key so forwarded emails can be properly encrypted.
+
+{% endblock %}