As you seem to be well aware, in a situation like yours, setting the return path to something that you control, is definitely important. As to how to do that, it should be as easy as simply reinjecting the email into postfix with the modified return path.

You can look into how to make postfix deliver your email to this command:

| sendmail -f postmaster@example.com username@stranger.net


I am just not sure if you can do that in the virtual_alias_maps table.

Forwarding mail to different mail system in Postfix
Dispatch creates hash tables only for lists of length larger than 3. Is there any way to force MMA to create hash tables even for lists of length smaller than 4?

Table[Head@Dispatch@Table[Rule[a[i], i], {i, n}], {n, 5}]
(* {List, List, List, Dispatch, Dispatch} *)

