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 188.8.131.52.
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:
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.