If you want to create a Pattern Filter that will block anything for non approved Top-Level Domains (TLDs), you can use the following in  Filter Rules > Pattern Filtering > Blacklisted Patterns:   


Filter Expression:  Matches Regular Expression

Value:    (.+\@)(?!(.+\.(com|gov|net|org|edu)\b))

edit the part in bold to match what you want


Check Apply to Headers -only-, and add "EnvelopeFrom" to the list of headers.


Rule Type:  Soft blacklist


Score:  15 (or similar)


What this will do is check the "Return-Path:" headers and flag them if they are not (?!) in the .com/.gov/.net/etc after the @ symbol

==> You will have to customize (com|gov|net|org|edu) to fix your needs


Unlike System Setup > Mail Relay > Sender Controls > Blacklisted Top Level Domains (TLDs), you can whitelist around this (even Whitelisted IP won't get around a Blacklisted TLD).  

You still may want to blacklist top-level domains for performance reasons and/or to keep their stuff out of Quarantine Reports.