queue-fs: mkdir: Permission denied

[morille]

We have smtpd running fine on a dedicated machine for more than 158 days, and today around 1PM all senders started receiving a "421 4.3.0 Temporary Error" and could not send email anymore.
The machine has no workload, plenty of ram and disk space.
Restarting smtpd fixed it.

Extract from mail.err

Dec  3 13:10:38 mail2 smtpd[3856037]: warn: queue-fs: mkdir: Permission denied
Dec  3 13:10:56 mail2 smtpd[3856037]: warn: queue-fs: mkdir: Permission denied
Dec  3 13:11:47 mail2 smtpd[3856037]: warn: queue-fs: mkdir: Permission denied
Dec  3 13:17:59 mail2 smtpd[3856037]: warn: queue-fs: mkdir: Permission denied
Dec  3 13:18:23 mail2 smtpd[3856037]: warn: queue-fs: mkdir: Permission denied
Dec  3 13:19:43 mail2 smtpd[3856037]: warn: queue-fs: mkdir: Permission denied
Dec  3 13:19:54 mail2 smtpd[3856037]: warn: queue-fs: mkdir: Permission denied
Dec  3 13:20:11 mail2 smtpd[3856037]: warn: queue-fs: mkdir: Permission denied
...

Extract from mail.log

Dec  3 14:22:07 mail2 smtpd[3856036]: b2162659e214a524 smtp connected address=XX.XX.XX.XX host=a.b.c.d
Dec  3 14:22:07 mail2 smtpd[3856036]: b2162659e214a524 smtp tls ciphers=TLSv1.3:TLS_AES_256_GCM_SHA384:256
Dec  3 14:22:07 mail2 smtpd[3856036]: b2162659e214a524 smtp authentication [email protected] result=ok
Dec  3 14:22:07 mail2 smtpd[3856037]: warn: queue-fs: mkdir: Permission denied
Dec  3 14:22:07 mail2 smtpd[3856036]: b2162659e214a524 smtp failed-command command="MAIL FROM:<[email protected]> BODY=8BITMIME SIZE=2951" result="421 4.3.0 Temporary Error"
Dec  3 14:22:07 mail2 smtpd[3856036]: b2162659e214a524 smtp disconnected reason=quit