When the destination server cannot be reached usually the origin server will try again for a number of hours before giving up (and in the meanwhile it may send

When the destination server cannot be reached usually the origin server will try again for a number of hours before giving up (and in the meanwhile it may send one or more annoying messages to the sender). I wouldn't do that though. I think that it is better to have a "regular" server (gmail etc.) handle it and have your server fetch the mail from it, without any risk of losing emails. I have a setup with fetchmail+postfix+dovecot that works very nicely.