#1125132 exim4: Cannot send email to smtp.themessagingco.com.au when using CGNAT address

Package:
exim4
Source:
exim4
Submitter:
Arthur Marsh
Date:
2026-01-16 06:15:01 UTC
Severity:
normal
#1125132#5
Date:
2026-01-09 13:00:24 UTC
From:
To:
Dear Maintainer,

*** Reporter, please consider answering these questions, where appropriate ***

   * What led up to the situation?

Switching ISP's to one that uses CGNAT IPv4 addresses.

exim4 messages like from reportbug gave the following error:

H=sep-kakadu01.au-east.atmailcloud.com [52.65.159.49]:
SMTP timeout after initial connection: Connection timed out

   * What exactly did you do (or not do) that was effective (or
     ineffective)?

I paid for a routable IPv4 address with the ISP and then experienced:

2026-01-09 21:21:58 1ve92e-000000000vz-3GgJ => arthur.marsh@internode.on.net R=s
marthost T=remote_smtp_smarthost H=smtp-tmc.pc5.atmailcloud.com [13.237.144.95]
X=TLS1.3:ECDHE_SECP256R1__RSA_PSS_RSAE_SHA256__AES_256_GCM:256 CV=yes DN="CN=*.t
hemessagingco.com.au" A=plain K C="250- 377 byte chunk, total 377\\n250 OK id=1v
eA61-00000000CT9-3wnP"

   * What was the outcome of this action?
   * What outcome did you expect instead?

Note that Thunderbird could successfully send smtp email to
smtp.themessagingco.com.au even with a CGNAT IP address.
#1125132#10
Date:
2026-01-09 13:44:06 UTC
From:
To:
Did you also pay for useable SMTP service?

That is a successful delivery to smtp-tmc.pc5.atmailcloud.com.

Greetings
Marc
#1125132#15
Date:
2026-01-16 03:59:08 UTC
From:
To:
Dear Maintainer,

*** Reporter, please consider answering these questions, where appropriate ***

   * What led up to the situation?

After trying what was suggested in:
https://unix.stackexchange.com/questions/729168/send-email-via-exim4-using-port-587
I was able to send outgoing SMTP mail to the smarthost on port 587

Maybe it should be documented that outgoing connections to the smarthost on
port 25 may be blocked, and where to change the configuration to match what
the smarthost supports?

   * What exactly did you do (or not do) that was effective (or
     ineffective)?
   * What was the outcome of this action?
   * What outcome did you expect instead?

*** End of the template - remove these template lines ***
#1125132#20
Date:
2026-01-16 06:13:16 UTC
From:
To:
I think that IS documented, and I am not too fond of making the exim
documnetation an "how do self-host your mail server on a connection that
your ISP doesnt want you to run a mail server on" tutorial.


Greetings
Marc