Relay

ISMail may optionally be configured to cooperate with other mail servers (in a group) in order to increase reliability or balance load by 'relaying' messages. The other mail servers may either be based on ISMail or using another SMTP server.

Relaying is defined by the type of mail server (primary or secondary) and controlled by DNS MX records and by mail server configuration.

Primary and Secondary Servers

Within a group of cooperating mail servers, the server which contains the mail accounts is called the 'primary' server. There is generally only one primary server for any given domain name.

The server(s) which support the primary server (by relaying messages to it) are called 'secondary' servers. Secondary servers do not contain mail accounts for domains on the primary server(s). There may be any number of secondary servers supporting domains on a primary server.

Mail for a domain on a primary server will generally (but not always) be sent directly to a primary server if it is operational. If a primary server is non-operational, mail will be sent to one of the secondary servers for later relay to the primary server.

DNS MX Records

The DNS MX records for a domain generally define the primary and secondary servers for the domain by specifying the precedence or priority of the servers for receiving messages for that domain. Therefore, the priority of the primary server for a domain is higher than that of the secondary server(s) for that domain.

In other words, the MX record for a domain on a primary server must have a higher priority (lower number) than the MX records for the domain on any secondary server(s).

Mail Server Configuration

ISMail may be configured as a primary server, a secondary server, or both at the same time (although it will be must act strictly as primary or as a secondary, but not both, for the domain(s) of any other specific mail server).

When configured as a primary mail server, ISMail will contain mail accounts for one or more domains, and accept messages for these accounts which are forwarded to it from the designated secondary server(s); it will also periodically query the secondary server(s) to determine if any are temporarily storing messages for any of the ISMail accounts.

When configured as a secondary mail server, ISMail will not contain mail accounts for the domains on the primary mail server(s) but it will accept and temporarily store messages for accounts in those domains; it will periodically attempt to send these messages to the primary server(s), and will respond to queries from the primary server(s) for those messages.

Configuring ISMail as a Primary Server (with Secondary servers):

To configure ISMail as a primary mail server with one or more secondary servers, setup the DNS records as described above. Then:

1. On the 'Relay' tab, click the 'Add' button next to 'These hosts are secondaries for this server' to add each secondary server.
2. Once all secondary servers have been added, click 'Apply'.

Configuring ISMail as a Secondary Server (for Primary servers);

To configure ISMail as a secondary mail server for one or more primary servers, setup the DNS records as described above. Then:

1. On the 'Relay' tab, click the 'Add' button next to 'This servers is a secondary for these hosts' to add each primary server.
2. Once all primary servers have been added, click 'Apply'.