diff --git a/CHANGELOG.md b/CHANGELOG.md
index d7af3644..187230fb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,9 @@
 - Update iroh-relay to 0.35.0
   ([#650](https://github.com/chatmail/relay/pull/650))
 
+- filtermail: accept mails from Protonmail
+  ([#616](https://github.com/chatmail/relay/pull/655))
+
 - Ignore all RCPT TO: parameters
   ([#651](https://github.com/chatmail/relay/pull/651))
 
diff --git a/chatmaild/src/chatmaild/filtermail.py b/chatmaild/src/chatmaild/filtermail.py
index 474d7a88..dd140d69 100644
--- a/chatmaild/src/chatmaild/filtermail.py
+++ b/chatmaild/src/chatmaild/filtermail.py
@@ -83,8 +83,14 @@ def check_openpgp_payload(payload: bytes):
     return False
 
 
-def check_armored_payload(payload: str):
-    prefix = "-----BEGIN PGP MESSAGE-----\r\n\r\n"
+def check_armored_payload(payload: str, outgoing: bool):
+    """Check the armored PGP message for invalid content.
+
+    :param payload: the armored PGP message
+    :param outgoing: whether the message is outgoing or incoming
+    :return: whether the message is a valid PGP message
+    """
+    prefix = "-----BEGIN PGP MESSAGE-----\r\n"
     if not payload.startswith(prefix):
         return False
     payload = payload.removeprefix(prefix)
@@ -96,6 +102,17 @@ def check_armored_payload(payload: str):
         return False
     payload = payload.removesuffix(suffix)
 
+    # Disallow comments in outgoing messages
+    version_comment = "Version: "
+    if payload.startswith(version_comment):
+        version_line = payload.splitlines()[0]
+        payload = payload.removeprefix(version_line)
+        if outgoing:
+            return False
+
+    while payload.startswith("\r\n"):
+        payload = payload.removeprefix("\r\n")
+
     # Remove CRC24.
     payload = payload.rpartition("=")[0]
 
@@ -131,7 +148,7 @@ def is_securejoin(message):
     return True
 
 
-def check_encrypted(message):
+def check_encrypted(message, outgoing=True):
     """Check that the message is an OpenPGP-encrypted message.
 
     MIME structure of the message must correspond to <https://www.rfc-editor.org/rfc/rfc3156>.
@@ -158,7 +175,7 @@ def check_encrypted(message):
             if part.get_content_type() != "application/octet-stream":
                 return False
 
-            if not check_armored_payload(part.get_payload()):
+            if not check_armored_payload(part.get_payload(), outgoing=outgoing):
                 return False
         else:
             return False
@@ -241,7 +258,7 @@ class OutgoingBeforeQueueHandler:
         logging.info(f"Processing DATA message from {envelope.mail_from}")
 
         message = BytesParser(policy=policy.default).parsebytes(envelope.content)
-        mail_encrypted = check_encrypted(message)
+        mail_encrypted = check_encrypted(message, outgoing=True)
 
         _, from_addr = parseaddr(message.get("from").strip())
 
@@ -301,7 +318,7 @@ class IncomingBeforeQueueHandler:
         logging.info(f"Processing DATA message from {envelope.mail_from}")
 
         message = BytesParser(policy=policy.default).parsebytes(envelope.content)
-        mail_encrypted = check_encrypted(message)
+        mail_encrypted = check_encrypted(message, outgoing=False)
 
         if mail_encrypted or is_securejoin(message):
             print("Incoming: Filtering encrypted mail.", file=sys.stderr)
diff --git a/chatmaild/src/chatmaild/tests/test_filtermail.py b/chatmaild/src/chatmaild/tests/test_filtermail.py
index d11f34e9..e39f4a09 100644
--- a/chatmaild/src/chatmaild/tests/test_filtermail.py
+++ b/chatmaild/src/chatmaild/tests/test_filtermail.py
@@ -241,8 +241,9 @@ def test_cleartext_passthrough_senders(gencreds, handler, maildata):
 
 
 def test_check_armored_payload():
-    payload = """-----BEGIN PGP MESSAGE-----\r
-\r
+    prefix = "-----BEGIN PGP MESSAGE-----\r\n"
+    comment = "Version: ProtonMail\r\n"
+    payload = """\r
 wU4DSqFx0d1yqAoSAQdAYkX/ZN/Az4B0k7X47zKyWrXxlDEdS3WOy0Yf2+GJTFgg\r
 Zk5ql0mLG8Ze+ZifCS0XMO4otlemSyJ0K1ZPdFMGzUDBTgNqzkFabxXoXRIBB0AM\r
 755wlX41X6Ay3KhnwBq7yEqSykVH6F3x11iHPKraLCAGZoaS8bKKNy/zg5slda1X\r
@@ -278,16 +279,25 @@ UN4fiB0KR9JyG2ayUdNJVkXZSZLnHyRgiaadlpUo16LVvw==\r
 \r
 """
 
-    assert check_armored_payload(payload) == True
+    commented_payload = prefix + comment + payload
+    assert check_armored_payload(commented_payload, outgoing=False) == True
+    assert check_armored_payload(commented_payload, outgoing=True) == False
+
+    payload = prefix + payload
+    assert check_armored_payload(payload, outgoing=False) == True
+    assert check_armored_payload(payload, outgoing=True) == True
 
     payload = payload.removesuffix("\r\n")
-    assert check_armored_payload(payload) == True
+    assert check_armored_payload(payload, outgoing=False) == True
+    assert check_armored_payload(payload, outgoing=True) == True
 
     payload = payload.removesuffix("\r\n")
-    assert check_armored_payload(payload) == True
+    assert check_armored_payload(payload, outgoing=False) == True
+    assert check_armored_payload(payload, outgoing=True) == True
 
     payload = payload.removesuffix("\r\n")
-    assert check_armored_payload(payload) == True
+    assert check_armored_payload(payload, outgoing=False) == True
+    assert check_armored_payload(payload, outgoing=True) == True
 
     payload = """-----BEGIN PGP MESSAGE-----\r
 \r
@@ -295,7 +305,8 @@ HELLOWORLD
 -----END PGP MESSAGE-----\r
 \r
 """
-    assert check_armored_payload(payload) == False
+    assert check_armored_payload(payload, outgoing=False) == False
+    assert check_armored_payload(payload, outgoing=True) == False
 
     payload = """-----BEGIN PGP MESSAGE-----\r
 \r
@@ -303,7 +314,8 @@ HELLOWORLD
 -----END PGP MESSAGE-----\r
 \r
 """
-    assert check_armored_payload(payload) == False
+    assert check_armored_payload(payload, outgoing=False) == False
+    assert check_armored_payload(payload, outgoing=True) == False
 
     # Test payload using partial body length
     # as generated by GopenPGP.
@@ -345,4 +357,5 @@ myLbG7cJB787QjplEyVe2P/JBO6xYvbkJLf9Q+HaviTO25rugRSrYsoKMDfO8VlQ\r
 =6iHb\r
 -----END PGP MESSAGE-----\r
 """
-    assert check_armored_payload(payload) == True
+    assert check_armored_payload(payload, outgoing=False) == True
+    assert check_armored_payload(payload, outgoing=True) == True