5.3.4 Reliable Mail Transmission

To transmit a message, a sender-SMTP determines the IP address of the target host from the destination address in the envelope. Specifically, it maps the string to the right of the "@" sign into an IP address. This mapping or the transfer itself may fail with a soft error, in which case the sender- SMTP will requeue the outgoing mail for a later retry, as required in Section 5.3.1.1.

When it succeeds, the mapping can result in a list of alternative delivery addresses rather than a single address, because of (a) multiple MX records, (b) multihoming, or both. To provide reliable mail transmission, the sender-SMTP MUST be able to try (and retry) each of the addresses in this list in order, until a delivery attempt succeeds. However, there MAY also be a configurable limit on the number of alternate addresses that can be tried. In any case, a host SHOULD try at least two addresses.

The following information is to be used to rank the host addresses:

1. Multiple MX Records -- these contain a preference indication that should be used in sorting. If there are multiple destinations with the same preference and there is no clear reason to favor one (e.g., by address preference), then the sender-SMTP SHOULD pick one at random to spread the load across multiple mail exchanges for a specific organization; note that this is a refinement of the procedure in [DNS:3].

2. Multihomed host -- The destination host (perhaps taken from the preferred MX record) may be multihomed, in which case the domain name resolver will return a list of alternative IP addresses. It is the responsibility of the domain name resolver interface (see Section 6.1.3.4 below) to have ordered this list by decreasing preference, and SMTP MUST try them in the order presented.

DISCUSSION:

Although the capability to try multiple alternative addresses is required, there may be circumstances where specific installations want to limit or disable the use of alternative addresses. The question of whether a sender should attempt retries using the different addresses of a multihomed host has been controversial. The main argument for using the multiple addresses is that it maximizes the probability of timely delivery, and indeed sometimes the probability of any delivery; the counter argument is that it may result in unnecessary resource use.

Note that resource use is also strongly determined by the sending strategy discussed in Section 5.3.1.