Setting Up Peering

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IMail Server lets you set up "peer" servers to allow users for a specific domain to be spread across multiple computers. This can be used when the mail traffic on your IMail Server system becomes heavy enough to slow down mail processing. How much traffic your mail server can handle depends on your computer's hardware configuration.

How Peering Works

If you have two systems set up as peer servers: both systems have IMail Server installed and each has a portion of the user database for a single domain. When mail arrives for a user in the domain (i.e. user@ipswitch.net), the sending server does a DNS lookup and gets the host name and address of one of peer servers. The mail is sent to one of the peer 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 server. If it finds the user, it forwards the mail.

Note: Do not select the Disable SMTP VRFY command (on the SMTP Security tab) when using peer servers. A peer server needs to use this command to verify a user that is on the other peer.

If either peer server is down, the other peer server receives and holds mail for it until the first server comes back up.

Setup Procedure

1. Install a licensed copy of IMail Server Version 8.10 or later on each system that will function as a peer mail server.
2. In your DNS server, add MX records for the peer servers. (see the example that follows this procedure.)
3. In the hosts file on each of the mail servers, make entries for all the mail servers.
4. On each mail server, use IMail Administrator to set up the Peer List as follows (assuming Tools > Use Wizards is cleared):
   • Select localhost in the left panel. On the General tab, click Add Host. The Virtual Host Configuration dialog box appears.
   • In the Virtual Host Configuration dialog box, select the mail host for which you want to add peer servers. This should be an actual host, not a virtual host.
   • Click Peer List in the lower right of the dialog box. The Peer List dialog box appears.
   • In the Peer List dialog box, enter an IP address and click Add to add it to the peer list.
   • Make sure the peer list exists on each computer that receives mail for a domain.

     Note: Do not add the IP address of the local system to the peer list; you enter only the other peers. (see the example that follows this procedure.)

In the Virtual Host Configuration dialog box for each server, in the Alias box, make sure the primary domain for sending and receiving mail (for example, ipswitch.net) is the only entry. The entry in the Alias box, must be the same on all servers. This alias cannot be a primary domain associated with a particular host. Do not enter IP addresses in the Alias box.

Example of Setting Up Peer Servers

Suppose you have one domain (called ipswitch.net) and three servers. All three servers accept incoming mail on the same priority and all have a portion of the user database. You would make the following entries in your DNS:

DNS entries:

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ipswitch.net
 
IN   MX 10 mail1.ipswitch.net 
IN   MX 10 mail2.ipswitch.net 
IN   MX 10 mail3.ipswitch.net
 
mail1.ipswitch.net      IN  A  1.1.1.1 
mail2.ipswitch.net      IN  A  2.2.2.2 
mail3.ipswitch.net      IN  A  3.3.3.3 
 

You create the following peer lists in the IMail Server software on the three servers:

Peer list on mail1:

2.2.2.2 
3.3.3.3 
 

Peer list on mail2:

1.1.1.1 
3.3.3.3 

Peer list on mail3:

1.1.1.1 
2.2.2.2 

On each of the three computers, make sure:

• The domain (in the form ipswitch.net) is the only entry in the Alias box in the Virtual Host Configuration dialog box.
• The Default Mail Host text box on the SMTP tab is blank when using peer lists.

  Note: You cannot use the "nobody" alias, to direct mail for non-existent users, if you use peering.

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