Adjustable Mail Headers

Some users find it confusing when they click "reply-to" and the mail client doesn't reply to the sender of the mail but the whole mailing list. This is a known feature and I understand that you usually don't want to have the sender's address unencrypted in the headers.

However, this is a usability vs security consideration, which is why in this merge request I added the option to set the reply-to header to the sender's mail address (default is off).

For lists where usability is more important it can be considered to set this value to true. The recipients will then receive e-mails where the "Reply-To" header will contain the unencrypted address of the sender and thus reply to the sender when clicking "reply-to" in a client. Additionally the "From" header will contain munched version (e.g. sender@example.org via <list>)

I know that this could be a quite controversial feature, so I am open to discuss/get denied xD