Если нужны квоты на пользователя:

cat /usr/local/etc/dovecot/dovecot_users

admin::1002:1002::/home/admin::userdb_quota_rule=*:bytes=5M:messages=1000

mcedit /usr/local/etc/dovecot/dovecot.conf

...
userdb {
  driver = passwd-file
  args = /usr/local/etc/dovecot/dovecot_users
}
userdb {
  driver = passwd
}
...