If your Dovecot fails to reload/restart after upgrade, chances are that your kernel is too old. But you can try and fix this problem with simple step bellow.
This was in syslog log:
systemd[20406]: dovecot.service: Failed at step NAMESPACE spawning /usr/bin/doveadm: No such file or directory dovecot.service: Control process exited, code=exited status=226 systemd[1]: Reload failed for Dovecot IMAP/POP3 email server.
This was Dovecot service status:
● dovecot.service - Dovecot IMAP/POP3 email server Loaded: loaded (/lib/systemd/system/dovecot.service; enabled; vendor preset: enabled) Active: active (running) (Result: exit-code) since Tue 2021-09-07 19:31:12 CEST; 1 months 28 days ago Docs: man:dovecot(1) Process: 20406 ExecReload=/usr/bin/doveadm reload (code=exited, status=226/NAMESPACE)
To solve, you can try and edit dovecot service script and add coment to PrivateTmp variable.
root@mailmachine:~# vi /lib/systemd/system/dovecot.service ... [Service] Type=forking ExecStart=/usr/sbin/dovecot PIDFile=/var/run/dovecot/master.pid ExecReload=/usr/bin/doveadm reload ExecStop=/usr/bin/doveadm stop #PrivateTmp=true NonBlocking=yes # Enable this if your systemd is new enough to support it: #ProtectSystem=full ...