dovecot unable to create files in /var/mail

The fix is to add postfix to the mail group in /etc/group
For example

mail:x:8:postfix

However this should be done by the package install script

I meant dovecot, not postfix. My bad.

Working on too many systems at once.

Thank you for taking the time to report this bug and helping to make Ubuntu better.

Importance -> Medium as there is a workaround available.

Without further research I can't confirm that this is definitely the right thing to do by default for all users. Unfortunately this is a change that can't be unwound easily, so I'd really prefer to see the bug resolved in Debian first unless it can be confirmed for certain that this is the right thing to do in Ubuntu. I'll add this to the server team's backlog, but I'm not sure we'll get to it soon.

Some additional research that touches on this same problem in Ubuntu:

https://www.softprayog.in/troubleshooting/dovecot-imap-error-open-var-mail-username-failed
https://wiki2.dovecot.org/MailLocation/mbox#A.2BAC8-var.2BAC8-mail.2BAC8_dotlocks
https://serverfault.com/questions/814398/dovecot-missing-w-perm-var-mail-were-not-in-group-8mail
https://askubuntu.com/questions/118416/dovecot-cant-compact-mail-folder-var-mail-username

There are a few different ways to set up where Dovecot delivers emails, but /var/mail is one of the more standard locations and perhaps configuring for it should be made more streamlined. I agree with Robie that this work would best be done upstream.

The workarounds are pretty widely documented, so I also agree with him this is not urgent to fix, and should remain in the backlog for now.

Dropping server-next tag. There are multiple ways to address this, and we prefer to see Debian set the direction for it.