Problem

I found the following error when restarting dovecot on some servers

Restarting IMAP/POP3 mail server: dovecotWarning: mail_extra_groups setting was often used insecurely so it is now deprecated, use mail_access_groups or mail_privileged_group instead

as the error suggests you need to use another variable in the dovecot.conf other than mail_extra_groups as it is now deprecated.

Resolution

Simply edit dovecot.conf

vi /etc/dovecot/dovecot.conf

replace mail_extra_groups

mail_extra_groups = mail

with mail_access_groups

mail_access_groups = mail

restart dovecot

/etc/init.d/dovecot restart

service dovecot restart

Now the error should no longer appear