4.5.4.1 Sending Strategy

   The general model for an SMTP client is one or more processes that
   periodically attempt to transmit outgoing mail.  In a typical system,
   the program that composes a message has some method for requesting
   immediate attention for a new piece of outgoing mail, while mail that
   cannot be transmitted immediately MUST be queued and periodically
   retried by the sender.  A mail queue entry will include not only the
   message itself but also the envelope information.

   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.  The
   parameters to the retry algorithm MUST be configurable.

 4yz   Transient Negative Completion reply
      The command was not accepted, and the requested action did not
      occur.  However, the error condition is temporary and the action
      may be requested again.  The sender should return to the beginning
      of the command sequence (if any).  It is difficult to assign a
      meaning to "transient" when two different sites (receiver- and



Klensin                     Standards Track                    [Page 42]

RFC 2821             Simple Mail Transfer Protocol            April 2001


      sender-SMTP agents) must agree on the interpretation.  Each reply
      in this category might have a different time value, but the SMTP
      client is encouraged to try again.  A rule of thumb to determine
      whether a reply fits into the 4yz or the 5yz category (see below)
      is that replies are 4yz if they can be successful if repeated
      without any change in command form or in properties of the sender
      or receiver (that is, the command is repeated identically and the
      receiver does not put up a new implementation.)