The sender MUST delay retrying a particular destination after one
attempt has failed. In general, the retry interval SHOULD be at
least 30 minutes; however, more sophisticated and variable strategies
will be beneficial when the SMTP client can determine the reason for
non-delivery.
Retries continue until the message is transmitted or the sender gives
up; the give-up time generally needs to be at least 4-5 days. It MAY
be appropriate to set a shorter maximum number of retries for non-
delivery notifications and equivalent error messages than for
standard messages. The parameters to the retry algorithm MUST be
configurable.