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Setting Up "Peer" IMail Servers

IMail Server lets you set up "peer" servers to allow users for a specific domain to be spread across multiple physical 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 will depend on your computer's hardware configuration.

How Peering Works

Suppose you have two systems set up as peer servers: both systems have IMail Server installed and each server has a portion of the user database for a single domain. When mail comes in for a user in the domain (for example, user@ipswitch.net), the sending server does a DNS lookup and gets the host name and address of one of the two 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 will receive and hold mail for it until the first server comes back up.

Setup Procedure

You will need to purchase and install a copy of IMail Server Version 4.0.3 or later on each peer server.

To add one or more peer servers for an IMail Server domain:

  1. Install a licensed copy of IMail Server Version 4.0.3 or later on each system that will function as a peer mail server.
  2. In your DNS zone file, add MX records for the peer servers. (See the example that follows this procedure.)
  3. In the hosts file on each of system, make entries for all the systems.
  4. On each system, use IMail Administrator to set up the Peer List as follows:
    • Select "localhost" in the left panel. On the General tab, click the Add Host button. 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 the Peer List button in the lower right of the dialog box. The Peer List dialog box appears.
    • In the "Peer List" dialog box, enter the IP addresses of the peer servers. IP addresses for a peer server should be for actual (not virtual) hosts. Enter an IP address into the text boxes and click Add to add it to the peer list.

      Make sure that the peer list exists on each computer that receives mail for a domain. Note that you 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.)

  5. In the "Virtual Host Configuration" dialog box for each mail server, in the Alias box, make sure the primary domain for sending and receiving mail (for example, ipswitch.net) is the only entry. Note that this alias cannot be a primary domain associated with a particular host. Do not enter IP addresses in the Alias box.
  6. For each mail server, make sure the Default Mail Host text box (on the SMTP tab) is empty when using peer lists.

Example of Setting Up Peer Servers

For example, 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: 

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

In the hosts file on each of the three servers, make the three entries: 

1.1.1.1  mail1.ipswitch.net 
2.2.2.2  mail2.ipswitch.net 
3.3.3.3  mail3.ipswitch.net

On each of the three computers, make sure:


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