Mails coming from the FreePBX web site such as account confirmation of password changes are sent by "apache@localhost.localdomain", which could be rejected by some mailer configurations.

ruleset=check_rcpt, arg1=<...>, relay=dfw-02.800hosting.com [69.41.160.214], reject=553 5.1.8 <...>... Domain of sender address apache@localhost.localdomain does not exist

Suggestion: The sender address should match the visible envelope "To:" address (info at freepbx dot org).