[Dovecot] fchown failed

Hello

I occasionally get the following error in dovecot log I've replaced the username by xxxxxxx

/user is the user's home directories place, does this means it must be owned by the "mail" group ?

Sep 14 09:20:48 IMAP(xxxxxxx): Error: fchown(/user/xxxxxxx/.imap/INBOX/dovecot.index.tmp, -1, 8(mail)) failed: Operation not permitted (egid=3420(math), group based on /var/mail/xxxxxxx)

Thanks