Rule Source: Header

Header: Return-Path

Rule Type: Regular-Expression

Weight: (variable based upon your Low/Med/High weights)

Rule Text: .+\.link>$

.+\.mobi>$

.+\.in>$

.+\.pw>$

.+\.rocks>$

.+\.science>$

.+\.work>$

.+\.ninja>$

.+\.cricket>$
.+\.click>$

.+\.party>$

.+\.tk>$

(et cetera for each top-level domain you want to catch)

 

Using .+ instead of .* shouldn't make too much of a difference, but can in some unique circumstances, but the important missing piece was the ending > before the end of the string $. Not sure why it would work sometimes without with ending > and not other times, but I was able to confirm that was the problem.

 

The other thing to note is do not put a : at the end of your Header field as the Custom Rules assumes a : and putting one in that field will result in it never matching (as it will be looking for Return-Path::). 

 

The same RegEx above should work with the From or Return-Path fields if you would prefer using those.

Hope this saves someone else some head-scratching frustration in the future.