Update: Since I'm a stubborn person and want to get to the bottom of the problem, I did some more testing (leaving the clients blocked for an hour...).
I discovered that the problem is indeed due to IPv6 (at least on Debian 12..., I don't know about other systems).
I then managed to resolve the issue by COMPLETELY DISABLING IPV6 on the Debian server using this procedure:
# Edit the GRUB configuration file
sudo nano /etc/default/grub
# Find the GRUB_CMDLINE_LINUX line and add 'ipv6.disable=1' to it
GRUB_CMDLINE_LINUX="ipv6.disable=1"
# Save the file and exit the editor
sudo update-grub
# Reboot the system
sudo reboot
After doing this and verifying that the Debian server no longer has IPv6 (with the command "ip a | grep inet6"), SmarterMail began accepting connections from Outlook/MAPI clients again.
Now everything is working again with the new 9406 release.
So I ask SmarterMail team to further investigate IPv6 support in Debian 12 (and perhaps on other systems too...)
Gabriele Maoret - Head of SysAdmins and CISO at SERSIS
Currently manages 6 SmarterMail installations (1 in the cloud for SERSIS which provides services to a few hundred third-party email domains + 5 on-premise for customers who prefer to have their mail server in-house)