docker-mailserver: Spoof failure with valid account and sender address

@17Halbe introduced in #872 spoof protection but this fails with ldap accounts.

 Mar 11 09:30:44 mail postfix/submission/smtpd[1874]: NOQUEUE: reject: RCPT from unknown[172.16.38.1]: 553 5.7.1 <johan@domain.net>: Sender address rejected: not owned by user username@mail.domain.net; from=<johan@domain.net> to=<johan@other.net> proto=ESMTP helo=<macbook>

The email server has mail.domain.net as the hostname and the email address is johan@domain.net. It verifies with my username and mail host domain to verify the sender address.

This is incorrect because it should check if the email address matches what is in the ldap as the mail field or mailalias fields.