Migrating email servers
I'm reasonably sure that if you don't have a valid mail server, the mail will be lost.  I mitigate this by installing a mail server (Postfix) on the same server as CiviCRM, and use that to relay to the third-party server.  Not only is this more robust - Postfix will hold the e-mails that don't go through while your main mail server is down for testing - but you get much finer-grained control over e-mail.  For instance, throttling the number of e-mails per second.