Correct way is create basically a second mail server that relays mail to your first when down. See http://www.cyberciti.biz/faq/postfix-backup-mx-server-anti-spam/ for postfix example. I would think you could find something similar with iredmail. Never used it so not sure.

If your mail server is down a correctly configured mail server will retry to send the message every X hours. (Think 4 hours is common) and then finally give up after X days. (5 days I think) Some mail servers may or may not respond to sender informing them mail can’t be sent.

Instead of creating a secondary server just to get the emails and hold them back until the primary, you can not create another email server that accesses the same database and the same directory as the primary? Then when the primary return will access the same information saved by the secondary server and vice versa with the secondary recovery and data recording? Today my email server already accesses the database on a separate server. Only miss access to separate files and web access.

Thank you.