It would mean, that the mails will come to mail.abc.com, except the server is down (or even overloaded), If so, the next mail server would be used: mail-backup.abc.com, of if the entire site has a problem the mail-backup.otherside.com
And there they are !!!! and there they stay !!!! However, the normal user is not checking mail-backup.abc.com or mail-backup.otherside.com Actually, I not even want that the mails on these machines would be sort in into mailboxes for the users, ... it should be just here in a queue to delivery to mail.abc.com as soon as possible.
I did a postfix install with the following /etc/postfix/main.cf on the backup mail server. I guess it can be shortened much more, down to just a few lines, but I'm no expert there. Works for me. --- Snip --- myhostname = mail-backup.abc.com relay_domains = $mydestination abc.com smtp_recipient_restrictions = permit_mynetworks check_relay_domains message_size_limit = 50000000 queue_run_delay = 100 maximal_queue_lifetime = 31 minimal_backoff_time = 100 maximal_backoff_time = 40 qmgr_message_recipient_limit = 1000 mail_spool_directory = /var/mail canonical_maps = hash:/etc/postfix/canonical virtual_maps = hash:/etc/postfix/virtual relocated_maps = hash:/etc/postfix/relocated smtpd_sender_restrictions = hash:/etc/postfix/access transport_maps = hash:/etc/postfix/transport masquerade_exceptions = root queue_directory = /var/spool/postfix command_directory = /usr/sbin daemon_directory = /usr/lib/postfix mail_owner = postfix myorigin = abc.com masquerade_domains = abc.com mydestination = $myhostname, localhost.$mydomain defer_transports = disable_dns_lookups = no relayhost = mail.abc.com ---- Snip ---- /Stefan