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Q271545: XCON: Message Timeouts and Open Retry Timeouts

Article: Q271545
Product(s): Microsoft Exchange
Version(s): 5.5
Operating System(s): 
Keyword(s): exc55
Last Modified: 13-DEC-2000

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The information in this article applies to:

- Microsoft Exchange Server, version 5.5 
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SUMMARY
=======

This KB article discusses the differences between message timeouts, and Open
Retry timeouts.

MORE INFORMATION
================

As an example of how these two issues may be confusing, look at the following
two articles in the Microsoft Knowledge Base:

  Q157630 XCON: Adjustment of Connection Retry Values on Override Tab

  Q178539 XCON: How to Adjust Message Timeouts for the Exchange MTA

At first glance, these articles can seem contradictory; it is unclear whether
mail is returned undeliverable after 24 hours, or after seven days (in the case
of Normal priority mail).

Message timeouts define how long a message has to live within the message
transfer system before the system returns the message undeliverable. A concept
that is very similar to the Time to Live (TTL) value in IP packets. Open Retry
intervals, on the other hand, define the length of time during which the message
transfer agent (MTA) tries to connect to another MTA to deliver a message before
giving up, and then returning the message as undeliverable.

Consider these two different hypothetical scenarios, assuming the simplest
configuration of only two servers (MTA1 and MTA2):

Scenario 1:

MTA1 attempts to send a message to MTA2, but MTA2 is down. Because MTA1 cannot
open a connection to MTA2, MTA1 falls into a state of Open Retry, and the Max
Open Retry counter starts to increment. By default, the retry interval (the
length of time between connection attempts) is 600 seconds, or five minutes. By
default, MTA1 tries to make a connection to MTA2 144 times before it giving up,
and then returning any messages, which were queued for MTA2, to their
originators as undeliverable. 144 retries multiplied by 600 seconds is equal to
86,400 seconds, which is approximately 24 hours. Thus, if MTA1 has mail to send
to MTA2, and MTA2 is down for 24 hours, all mail, regardless of the priority,
that was bound for MTA2 is returned as undeliverable. For additional
information, click the article number below to view the article in the Microsoft
Knowledge Base:

  Q157630 XCON: Adjustment of Connection Retry Values on Override Tab

Scenario 2:

MTA1 has mail queued for MTA2, but before it completes the delivery of all the
mail, MTA1 shuts down; after a while, MTA1 is back online. The mail that is in
the MTA1 queue does not timeout until the MTA on MTA1 detects that the default
timeout period for the priority of a given message has elapsed. The defaults are
two days for High priority, seven days for Normal priority, and ten days for Low
priority messages. Thus, if MTA1 has mail queued for MTA2, and MTA1 shuts down
completely for five days, when MTA1 is back online, all of the High priority
mail that MTA1 had queued for all destinations is returned as undeliverable to
the message originators. However, messages with a Low or Normal priority remain
in the queue until they are delivered. For additional information, click the
article number below to view the article in the Microsoft Knowledge Base:

  Q178539 XCON: How to Adjust Message Timeouts for the Exchange MTA

The importance of this issue is determined by which server is down: the server
that has the mail queued (the Source), or the server that should receive the
mail (the Destination).

If you have more than two MTAs in the site, or more than one route, which a
message can use for delivery, then message rerouting factors into the equation.
Remember that the MTA can reroute mail that is queued to non-secured queues,
such as the X.400 Connector, Site Connector, Dynamic RAS Connector, and other
MTAs. The MTA cannot reroute mail that has been queued to secured queues, such
as the Internet Mail Service, and the Microsoft Exchange Connectors for MS Mail,
cc:Mail, Lotus Notes, GroupWise, SNADS, and PROFS.

Additional query words:

======================================================================
Keywords          : exc55 
Component         : MTA
Technology        : kbExchangeSearch kbExchange550 kbZNotKeyword2
Version           : :5.5
Issue type        : kbinfo

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