Email delivery issues with double forwarding to external addresses

[avatar1024]

To keep going with email delivery issues I have noticed the following started to happen recently:

domain1 and domain2 mentioned below are on two different servers managed as Cloudron instances (v8.2.3) and both use built-in SMTP.

mailbox@domain1.coop forwards to several people and to mailbox@domain2.coop.

mailbox@domain2.coop is monitored as a mailbox and also forwards to three people:
person1@gmail.com
person2@posteo.net
person3@domain2.coop (another mailbox on the same server)

When sending an email to mailbox@domain1.coop email get delivered to everyone except to:

1. person1@gmail.com with the following error:
   "Syntax error, cannot decode response. For more information, go to https://support.google.com/a/answer/3221692 and review RFC 5321 specifications. a640c23a62f3a-ab2c9415805si80260366b.277 - gsmtp"

2. person2@posteo.net with the following error:
   "521 5.5.2 mx01.posteo.de Error: bare <LF> received"

For person3@domain2.coop the email is "delivered" as just "Saved" according tot he Event Log.

However sending an email directly to mailbox@domain2.coop delivers fine to everyone.

I've also done other test an the same errors only appears when an email is forwarded twice (like in this example mailbox@domain1.coop > mailbox@domain2.coop > person outside of the server).

[avatar1024]

Anyone got a clue at what might be causing this? Happy to send logs or anything else that might help debug.

[NCKNE]

Not sure if this might be due to some SRS (https://en.wikipedia.org/wiki/Sender_Rewriting_Scheme) misinterpretation? I have set up similar scenarios on Cloudron, but switched to an external SMTP relay server - so not comparable per se. If not using SRS, you would run into SPF problems when forwarding the mails aka using a distribution group.

The "Error: bare <LF> received" message might hint at some other cause though, maybe the Haraka mail server used by Cloudron formatted something in a non-conforming way.

[avatar1024]

@NCKNE said in Email delivery issues with double forwarding to external addresses:

Not sure if this might be due to some SRS (https://en.wikipedia.org/wiki/Sender_Rewriting_Scheme) misinterpretation?

This is my feeling too...but I'm not sure.

@NCKNE said in Email delivery issues with double forwarding to external addresses:

I have set up similar scenarios on Cloudron, but switched to an external SMTP relay server

Does it work fine with your set-up?

[NCKNE]

@avatar1024 said in Email delivery issues with double forwarding to external addresses:

Does it work fine with your set-up?

Yes, using an external SMTP relay it works for me. And Google finally accepts incoming mails right away instead of delaying them for hours or even days...

[crazybrad]

I agree about using an external SMTP relay. In my case, Postmark has been flawless. Because they separate marketing from transactional messages, their deliverability seems better than their competition. I also set up a sub-account (in Postmark jargon, it's called a Server). By doing that, each domain's reputation is isolated. If someone behaves badly, it is easy to delete their server keeping the rest of your infrastructure intact. By adding a "domain-based" Sender Signature with DKIM and Return-Path, any email sender from that domain is permitted. It works really well!