Drup.org

The problem with the normal MX backup strategy is spam: http://blog.sembee.co.uk/archive/2007/02/02/38.aspx
Free service: http://www.junkemailfilter.com/spam/free_mx_backup_service.html
$30/year service: http://www.no-ip.com/services/managed_mail/backup_mail.html
$20/year service: http://www.dnsexit.com/Direct.sv?cmd=backupMx
free but no anti-spam: http://www.dyndns.com/support/kb/backupmx.html