SASL Authentication

==== Required information ====
  iRedAdmin-Pro    v1.6.0 (MySQL)
  MySQL
  CentOS 6.4 with kernel patch 2013-05-29
====

I believe this has to with SASL Authentication. Essentially, a program a client uses from a 3rd party company sends mail out to his clients as his email. It get's delivered to the clients however, he always receives a bounce notification for himself.

If I remove from smtpd_sender_restriction the option "reject_sender_login_mismatch" it works.

Is there an alternative that will only affect the user or domain rather than globally?

Thanks,
Neil