Add qualifier for qa testers at social media companies (#78)

* Add clarifying language to missing mail sections

* Remove custom domain section

This section is essentially the same as the BYOD section.

* Add deny list documentation

* RST formatting fixes

* Add instructions for social media company QA testers

help/missing_mail.rst

@@ -141,6 +141,8 @@ may be added to the deny list.
 
 Emails from social media and streaming services are blocked. Emails from these
 services have been used in ways that are against the :ref:`doc_terms_of_service`.
+If you are a QA tester working for a social media company or streaming service
+and wish to use Mailsac reach out to our support team.
 
 Contacting Support
 ------------------