Solved IPv6 SMTP Problems with Voicemail

[LogitComputer]

Hello,

I've posted this some times ago, but I ran into the same problem again:
- 3CX AI Version, self hosted, valid IPv4 and IPv6, DNS A and AAAA records correct
- own SMTP Server, hosted in data center, IPv4 and IPv6, DNS A and AAAA records correct, PTR correct too

Everythind works correct, alerts are coming in etc.

Unfortunally, only the Voicemail Mails with the attached WAV file doesnt work, in the eventlog:

Fehler
50026
28.10.2026 14:48
System Manager
Logit
The following error occurred when trying to send an email with subject Neue Sprachnachricht von 071xxxxxxx – 071xxxxxxx to [email protected] :. Eine vorhandene Verbindung wurde vom Remotehost geschlossen.

My workaround was to modifiy the "C:\Windows\System32\drivers\etc\hosts" file and put the DNS name of my Mailserver as IPv4, then, the Voicemails goes out.

Could anyone reproduce this issue or 3CX, could you investigate why just the Voicemail transmission fails?

FYI: there is no contact from 3CX to my Mail Server via IPv6 when the Voicemails should go out.

Thank you.

 

[NikosT_3CX]

@LogitComputer the way to troubleshoot this is :
1) remove the hosts files and reboot the Widnows machine
2) Setup the PBX in Verbose mode and start wireshark.
3) Leave the Vmail and ensure the user has " Send voicemail as attachment "

When done open the System Service log.
Check there as a first step thay you can see your IPv6 in the logs ( after the IPv4 ).
Then find the time the vmail was left.
Open pcap and filter with the IPv6 of your Mail Server as this is the destination.
Check the logs with the same time inside the pcap.

 

[LogitComputer]

Hello Nikos,

Thank you for your hint. I have a Mikrotik Router, and for IPv6, I had to "mangle" the MSS according to Mikrotik.
But for the bigger packets, I had to mangle the outbound too, so therefore I put a 2nd rule to change the MSS for outbound IPv6 traffic as well:

0 ;;; IPv6 MSS clamp 1452 outbound
chain=forward action=change-mss new-mss=1392 passthrough=yes protocol=tcp tcp-flags=syn out-interface=pppoe-out1 tcp-mss=1393-65535

1 ;;; IPv6 MSS clamp 1452 inbound
chain=forward action=change-mss new-mss=1392 passthrough=yes protocol=tcp tcp-flags=syn in-interface=pppoe-out1 tcp-mss=1393-65535

The log-mails passed before becaus the size of these were around 760 bytes, so below the MSS. The voicemails are bigger, therefore the issue with "only voicemail" because of the size.

After a reboot, all voicemails were transmitted via IPv6.Thank you.

Regards,

Josip