How long should a person have a mail system keep an outgoing message
if the mail system cannot form a connection to the destination
computer, and why?

Greetings,

In order to comply with RFC 2821, you should configure a mail
system to retry for at least 4-5 days.  Here is a quote from RFC
2821:

"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.

"A client SHOULD keep a list of hosts it cannot reach and
corresponding connection timeouts, rather than just retrying
queued mail items.

"Experience suggests that failures are typically transient (the
target system or its connection has crashed), favoring a policy of
two connection attempts in the first hour the message is in the
queue, and then backing off to one every two or three hours."

"Simple Mail Transfer Protocol", Copyright (C) The
Internet Society (2001)

A copy of this RFC may be found on the website of the Internet
Engineering Task Force (IETF) at the following address: 

URL: http://www.ietf.org/rfc/rfc2821.txt

Personally, I wouldn't retry for longer than 5 days, because most
network problems are resolved in much less time.  Depending on the
amount of traffic through your server, you may end up using a lot
of resources queueing messages that will never be delivered.


Additional Links:

The Sendmail homepage
http://www.sendmail.org

The Internet Mail Consortium (IMC)
http://www.imc.org

The Internet Engineering Task Force (IETF)
http://www.ietf.org


Search Strategy:

No searching, really.  Reading the Sendmail FAQ led me to the
Internet Mail Consortium's homepage, which led me to RFC 2821.


Best regards,
bikerman-ga

Thanks, this was good!