The simplest hack I can think of is: * Making a wildcard email address for domain, say @example.com making a filter that only allows sourcedomain@example.com

The simplest hack I can think of is: * Making a wildcard email address for domain, say @example.com making a filter that only allows sourcedomain@example.com (or user_sourcedomain@example.com) to accept mails from @sourcedomain Could replace it with hash in second part hashing whole sender's address with some secret but that might be annoying if you have to tell the email via phone and now it needs to be generated manually But > 4. Even if the original authorized party tries to email me from a different address at the same domain, that email will be handled as in (3). 5 Would likely bite you HARD as it isn't too uncommon practice to respond from other email if say you are contacting some generic alias (contact@) but get answer from (john.doe@) that handles your case.