The first step for the mailer at LOCAL is to issue a query for MX RRs for REMOTE. It is strongly urged that this step be taken every time a mailer attempts to send the message. The hope is that changes in the domain database will rapidly be used by mailers, and thus domain administrators will be able to re-route in-transit messages for defective hosts by simply changing their domain databases.
Certain responses to the query are considered errors:
Mailers are expected to do something reasonable in the face of an error. The behaviour for each type of error is not specified here, but implementors should note that different types of errors should probably be treated differently. For example, a response code of "non-existent domain" should probably cause the message to be returned to the sender as invalid, while a response code of "server failure" should probably cause the message to be retried later.
There is one other special case. If the response contains an answer which is a CNAME RR, it indicates that REMOTE is actually an alias for some other domain name. The query should be repeated with the canonical domain name.
If the response does not contain an error response, and does not contain aliases, its answer section should be a (possibly zero length) list of MX RRs for domain name REMOTE (or REMOTE's true domain name if REMOTE was a alias). The next section describes how this list is interpreted.