[feat] keep RCPT when recipient is MX

[madmuffin1]

(following up on #242, which is closed, but I believe that was a misunderstanding, I believe this is also what #249 is trying to achieve)

Scenario: I run a mail server. For a number of reasons, I need them to keep their setup as closely as it is as possible. But I am very unhappy with the current spam filter - and I was looking to use forwardemail mainly as spam filter.
Other use cases - from the other referenced issues: Having a fully setup mail server, that has to move to a different port (since getting an open port 25 is an issue with some providers)

So, let's assume on my server there is a mailbox foo@bar.com, and my MX is currently set to mx.bar.com.

If I replace my MX with forwardemails MX, there is no way to have forwardemail direct all mail to mx.bar.com, without the envelope RCPT being rewritten to foo@mx.bar.com - which is not a valid address on that server.

I would suggest allowing something like mx:mx.bar.com as value, so forwardemail knows that it should not rewrite the RCPT skip the MX lookup part and forward all traffic to that server.

Checklist

• I have searched through GitHub issues for similar issues.
• I have completely read through the README and documentation.