# display the groups user 'dovecot' is in
$groups dovecot
dovecot : dovecot mail
# as shown, user 'dovecot' is in group 'dovecot and group 'mail'

# display the group user 'mail' is in
$groups mail
mail : mail
# as shown, user 'mail' is in group 'mail'

# check who are the members of group 'mail' and I should see user 'dovecot' and user 'mail'
$cat /etc/group |grep mail
mail:8:dovecot
# however, I only see user 'dovecot' here

Is there a reason why user 'mail' is not shown as a member of group 'mail' in the last command?

Thanks!