Let me vote against this option (even though I'd appreciate it myself). When you make people read the messages on the website, you can see when someone has read (or not read) a message. Especially when discussing swaps it sometimes happens that people just don't have time for a few days to read messages (no problem at all), on the other hand it can happen that people have read them but decide not to react for a few days (that swap will go nowhere) ...
If the message is in the mail, you have no idea of knowing if it was read or not.
BTW : Xavier, we still don't seem to have the downvote button for suggestions