Add smtp_max_line_length_kb to accept non-conformant SMTP lines (fixes silent drops from e.g. Swann NVW-MR4K)

smtp2mqtt/CHANGELOG.md

@@ -1,3 +1,9 @@
+## [1.0.5] - 2026-08-26
+
+### Added
+
+- New configuration option `smtp_max_line_length_kb` to raise aiosmtpd's SMTP line-length limit above the RFC 5321 4.5.3.1.6 default, to accommodate devices (e.g. the Swann NVW-MR4K / MaxRanger4K security hub) that send a base64-encoded attachment as one unbroken line with no CRLF wrapping (violates RFC 2045 6.8), which previously caused the add-on to reject the message with `500 Line too long` and never publish it.
+
 ## [1.0.4] - 2024-06-24
 
 ### Changed

smtp2mqtt/DOCS.md

@@ -67,6 +67,7 @@ mqtt_pass:
 mqtt_topic: smtp2mqtt
 
 smtp_auth_required: false
+smtp_max_line_length_kb:
 
 smtp_relay_host: 
 smtp_relay_port: 25
@@ -122,6 +123,18 @@ If enabled, a simple dummy authentication is performed that always returns succe
 
 Default value: `false`
 
+### Option: `smtp_max_line_length_kb` (optional)
+
+Raises the maximum SMTP line length (in KB) that the add-on will accept during the `DATA` phase, instead of the RFC 5321 4.5.3.1.6 default of ~1000 octets.
+
+Some devices violate RFC 2045 6.8 by sending a base64-encoded attachment (e.g. a JPEG snapshot) as a single, unbroken line with no CRLF wrapping. Since aiosmtpd's default line length limit is only ~1000 bytes, it will reject any such line with `500 Line too long` and abort the connection - which means the add-on never receives (and can never publish) that message at all. This has been observed with the Swann NVW-MR4K / MaxRanger4K security hub's motion-alert emails, which always include such a snapshot.
+
+If you're seeing `500 Line too long (see RFC5321 4.5.3.1.6)` in the log (with `debug: true`) right before the sending device gives up, set this to a value comfortably larger than the base64-encoded size of the largest attachment you expect (base64 inflates size by ~4/3), e.g. `8192` for an ~6MB snapshot.
+
+Leave this unset to keep the standard, RFC-compliant line length limit - this only exists to accommodate non-conformant senders that can't be fixed on their end.
+
+Default value: *(unset - standard aiosmtpd line length limit applies)*
+
 ### Option: `smtp_relay_host` (optional)
 
 The host name or IP address of an SMTP server to relay the message through, in order to send it on its way.

smtp2mqtt/Dockerfile

@@ -1,13 +1,12 @@
-ARG BUILD_FROM
-FROM $BUILD_FROM
+FROM ghcr.io/home-assistant/base:3.24
 
 WORKDIR /app
 
 COPY start.sh smtp2mqtt.py /app/
 
 RUN chmod 755 /app/start.sh
 
-RUN apk add --no-cache python3 py3-pip && pip3 install paho-mqtt aiosmtpd
+RUN apk add --no-cache python3 py3-pip && pip3 install --break-system-packages paho-mqtt aiosmtpd
 
 ENTRYPOINT ["/app/start.sh"]
 

smtp2mqtt/config.yaml

@@ -2,15 +2,14 @@
 name: "SMTP to MQTT"
 description: "A simple, tiny, absolutely no-frills Python-based SMTP server that publishes mail to an MQTT topic then (optionally) passes the mail on to the next relay"
 url: "https://github.com/bcastellucci/addons"
-version: "1.0.4"
+version: "1.0.5"
 slug: "smtp2mqtt"
 arch:
   - aarch64
   - amd64
   - armhf
   - armv7
   - i386
-image: "bcastellucci/image-{arch}-smtp2mqtt"
 startup: application
 boot: auto
 map:
@@ -24,6 +23,7 @@ schema:
   mqtt_pass: password?
   mqtt_topic: str?
   smtp_auth_required: bool?
+  smtp_max_line_length_kb: int(1,65536)?
   smtp_relay_host: str?
   smtp_relay_port: port?
   smtp_relay_user: str?

smtp2mqtt/smtp2mqtt.py

@@ -12,6 +12,7 @@
 from email.policy import default
 from aiosmtpd.controller import Controller
 from aiosmtpd.smtp import AuthResult
+from aiosmtpd.smtp import SMTP as AiosmtpdSMTP
 from paho.mqtt import publish
 
 ##
@@ -34,6 +35,7 @@
     "MQTT_TOPIC": "smtp2mqtt",
     "PUBLISH_ATTACHMENTS": "False",
     "SAVE_ATTACHMENTS_DIR": None,
+    "SMTP_MAX_LINE_LENGTH_KB": None,
     "DEBUG": "False",
 }
 config = {
@@ -193,13 +195,54 @@ def dummy_auth_function(server, session, envelope, mechanism, auth_data):
     return AuthResult(success=True)
 
 
+def make_relaxed_line_length_controller(line_length_bytes):
+    """
+    Some devices (e.g. the Swann NVW-MR4K / MaxRanger4K security hub) attach a
+    base64-encoded JPEG snapshot as a single, unbroken line with no CRLF
+    wrapping, which violates RFC 2045 6.8. aiosmtpd enforces the RFC 5321
+    4.5.3.1.6 ~1000-octet SMTP line limit via SMTP.line_length_limit (which
+    also sizes the underlying asyncio.StreamReader buffer) and aborts the
+    DATA phase with "500 Line too long" as soon as it's hit, so such devices
+    can never successfully deliver a message with an attachment.
+
+    SMTP_MAX_LINE_LENGTH_KB lets an operator raise that limit for such
+    non-conformant senders. Returns a Controller subclass whose factory()
+    builds an SMTP protocol instance with the raised limit instead of
+    aiosmtpd's default of 1001 bytes.
+    """
+    class RelaxedLineLengthSMTP(AiosmtpdSMTP):
+        pass
+    RelaxedLineLengthSMTP.line_length_limit = line_length_bytes
+
+    class RelaxedLineLengthController(Controller):
+        def factory(self):
+            return RelaxedLineLengthSMTP(self.handler, **self.SMTP_kwargs)
+
+    return RelaxedLineLengthController
+
+
 if __name__ == "__main__":
     if (log.isEnabledFor(logging.DEBUG)):
         log.debug(", ".join([f"{k}={v}" for k, v in config.items()]), extra={'uuid': 'main thread'})
 
     loop = asyncio.new_event_loop()
     asyncio.set_event_loop(loop)
-    c = Controller(
+
+    controller_kwargs = {}
+    if config["SMTP_MAX_LINE_LENGTH_KB"]:
+        line_length_bytes = int(config["SMTP_MAX_LINE_LENGTH_KB"]) * 1024
+        controller_class = make_relaxed_line_length_controller(line_length_bytes)
+        # a single oversized line can be nearly the whole message, so make
+        # sure the overall DATA size cap isn't smaller than the line itself
+        controller_kwargs["data_size_limit"] = max(2 ** 25, line_length_bytes * 2)
+        log.info(
+            "Relaxing SMTP line length limit to %d bytes (SMTP_MAX_LINE_LENGTH_KB=%s)",
+            line_length_bytes, config["SMTP_MAX_LINE_LENGTH_KB"], extra={'uuid': 'main thread'},
+        )
+    else:
+        controller_class = Controller
+
+    c = controller_class(
         handler=SMTP2MQTTHandler(loop),
         loop=loop,
         hostname="0.0.0.0",
@@ -208,6 +251,7 @@ def dummy_auth_function(server, session, envelope, mechanism, auth_data):
         auth_required=config["SMTP_AUTH_REQUIRED"],
         #quite down the warnings when auth-required is false by letting this be true (we won't use it anyway, doesn't hurt to be true)...
         auth_require_tls=(not config["SMTP_AUTH_REQUIRED"]),
+        **controller_kwargs,
     )
     c.start()
     log.info("Running", extra={'uuid': 'main thread'})

smtp2mqtt/start.sh

@@ -21,7 +21,7 @@ export MQTT_HOST=$MQTT_HOST
 
 #grab the rest in a loop
 for var in \
-    smtp_auth_required \
+    smtp_auth_required smtp_max_line_length_kb \
     smtp_relay_host smtp_relay_port smtp_relay_user smtp_relay_pass smtp_relay_starttls smtp_relay_timeout_secs \
     mqtt_port mqtt_user mqtt_pass mqtt_topic \
     publish_attachments \