If you have McAfee Enterprise version then the solution is in this article -
When the option Or if recipient is not listed in LDAP is enabled under the recipient Check option of Email recipient authentication in Email and Web Security Appliance software, you see outbound emails blocked with the error code:
550 - Denied by policy.
Enabling recipient check Or if recipient is not listed in LDAP by default applies the check as a global policy. Ensure you apply this recipient check only for incoming SMTP traffic.
- The full details of how to do this are given immediately after that advice.
your internal mail server falling in Authentication\RBL so no need to worry simply create on group and create new mail policy for only your internal mail server and disabled sender authentication disabled for only outbound mail traffic for this group .