imap(user): Error: file_dotlock_create(/var/mail/user) failed: Permission denied (euid=1001(user) egid=1001(user) missing +w perm: /var/mail, we're not in group 8(mail), dir owned by 0:8 mode=0775) (set mail_privileged_group=mail)

nano /etc/dovecot/dovecot.conf
and insert:
mail_privileged_group=mail

chmod user:mail /var/mail/user

chown 0770 /var/mail/user