Multiple INBOXes? not me. All I know is I filter for my single address.
(Also never dared to use X-loop, using a different final destination mailbox instead.)
Anyway, with a fancy enough .procmailrc, you could certainly do
many->one->many. Note that some mail is “undisclosed recipients”, so
you need to bifurcate on “Received …for” headers.
Anyways everything is hunky dory, except if I am at a terminal at a
public library that blocks port 22 so I can’t use putty to read via
mutt on my shell account.
Whereupon all I can use is http(s) to webmail, which is like “driving
with the steering wheel locked”…