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

[Arthur Marsh]

Package: exim4
Version: 4.99.1-1
Severity: important

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 at 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.


-- Package-specific info:
Exim version 4.99.1 #2 built 21-Dec-2025 07:00:57
Copyright (c) University of Cambridge, 1995 - 2018
(c) The Exim Maintainers and contributors in ACKNOWLEDGMENTS file, 2007 - 2025
Hints DB:
 Using sqlite3
Support for: Local_Scan Exim_filter Sieve_filter crypteq iconv() IPv6 GnuTLS move_frozen_messages TLS_resume DANE DKIM DNSSEC ESMTP_Limits ESMTP_Wellknown Event I18N OCSP PIPECONNECT PRDR Queue_Ramp SOCKS SRS TCP_Fast_Open
Lookups (built-in): lsearch wildlsearch nwildlsearch iplsearch cdb dbm dbmjz dbmnz dnsdb dsearch nis nis0 passwd sqlite
Authenticators (built-in): cram_md5 external plaintext
Routers (built-in): accept dnslookup ipliteral manualroute redirect queryprogram
Transports (built-in): appendfile/maildir/mailstore autoreply lmtp pipe smtp
Fixed never_users: 0
Configure owner: 0:0
Size of off_t: 8
Configuration file search path is /etc/exim4/exim4.conf:/var/lib/exim4/config.autogenerated
Configuration file is /var/lib/exim4/config.autogenerated
# /etc/exim4/update-exim4.conf.conf
#
# Edit this file and /etc/mailname by hand and execute update-exim4.conf
# yourself or use 'dpkg-reconfigure exim4-config'
#
# Please note that this is _not_ a dpkg-conffile and that automatic changes
# to this file might happen. The code handling this will honor your local
# changes, so this is usually fine, but will break local schemes that mess
# around with multiple versions of the file.
#
# update-exim4.conf uses this file to determine variable values to generate
# exim configuration macros for the configuration file.
#
# Most settings found in here do have corresponding questions in the
# Debconf configuration, but not all of them.
#
# This is a Debian specific file

dc_eximconfig_configtype='smarthost'
dc_other_hostnames=''
dc_local_interfaces='127.0.0.1'
dc_readhost='internode.on.net'
dc_relay_domains=''
dc_minimaldns='false'
dc_relay_nets=''
dc_smarthost='smtp.themessagingco.com.au'
CFILEMODE='644'
dc_use_split_config='false'
dc_hide_mailname='true'
dc_mailname_in_oh='true'
dc_localdelivery='mail_spool'
mailname:localhost
# /etc/default/exim4
EX4DEF_VERSION=''

# 'combined' -	 one daemon running queue and listening on SMTP port
# 'no'       -	 no daemon running the queue
# 'separate' -	 two separate daemons
# 'ppp'      -   only run queue with /etc/ppp/ip-up.d/exim4.
# 'nodaemon' - no daemon is started at all.
# 'queueonly' - only a queue running daemon is started, no SMTP listener.
# setting this to 'no' will also disable queueruns from /etc/ppp/ip-up.d/exim4
QUEUERUNNER='combined'
# how often should we run the queue
QUEUEINTERVAL='30m'
# options common to quez-runner and listening daemon
COMMONOPTIONS=''
# more options for the daemon/process running the queue (applies to the one
# started in /etc/ppp/ip-up.d/exim4, too.
QUEUERUNNEROPTIONS=''
# special flags given to exim directly after the -q. See exim(8)
QFLAGS=''
# options for daemon listening on port 25
SMTPLISTENEROPTIONS=''

-- System Information:
Debian Release: forky/sid
  APT prefers experimental
  APT policy: (1, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 6.19.0-rc4+ (SMP w/12 CPU threads; PREEMPT)
Kernel taint flags: TAINT_USER
Locale: LANG=en_AU.UTF-8, LC_CTYPE=en_AU.UTF-8 (charmap=UTF-8), LANGUAGE=en_GB
Shell: /bin/sh linked to /usr/bin/dash
Init: sysvinit (via /sbin/init)

Versions of packages exim4 depends on:
ii  debconf [debconf-2.0]  1.5.91
ii  exim4-base             4.99.1-1
ii  exim4-daemon-light     4.99.1-1

exim4 recommends no packages.

exim4 suggests no packages.

-- debconf information:
* exim4/drec: