Suppose you have two systems with IMail installed on each system and you set up the two systems as peer servers. Each system has a portion of the user database for a single mail host.
When mail is sent to an e-mail address, the sending server does a DNS lookup to get the mail domain name and address associated with the e-mail address. If the IMail Server that is handling mail for the message is configured for peering, mail comes in for a user on the mail domain and the mail is directed to one of the peer mail servers.
If the user is found on the peer server, the mail is delivered. If not, the peer server does an "SMTP Verify" to see if the user exists on the other mail server. If the user is found in the user database, it forwards the mail. If either peer server is down, the other peer server receives and holds mail until the first server comes back up.
Note: When using peer servers, do not select the Disable SMTP "VRFY"Command on the Services > SMTP tab. Peer servers need to use this command to verify a user that is on the other peer.