The default value of `first_valid_uid` in *dovecot* has changed
In Red Hat Enterprise Linux 7, the default configuration of `first_valid_uid` in *dovecot* was changed to `1000` to match the system wide configuration specified as `UID_MIN` in the `/etc/login.defs` file. If a system has `UID_MIN` manually changed to `500` and is relying on *dovecot* default value, *dovecot* will not serve users with IDs lower than `first_valid_uid`.
As a consequence, if you have regular users with id less than `1000`, you have to update `first_valid_uid`. After you do this, *dovecot* will work as expected.