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possible reason
When the destination smtp server is using postfix and installed a free-ware named graylisting.
For those who doesn't know - greylisting is used on some mail servers to
tempfail first attempt of an email, asking the sending server to retry later.
In short (more details will follow) - Exchange 2003 SP2 failes to re-queue
messages sent to some servers that implement greylisting.
This does not happen all the time (some messages go through but sometimes it
fails).
When the problem happens, those emails are hidden in some kind of a black
hole, and the sender does not get an NDR nor Delay notification, even after
those timeouts expire.
Such messsages can remain "lost" for days or even weeks, until the SMTP or
Information Store service is restarted.
After a restart of SMTP service, Exchange suddenly finds those lost emails
(I guess they were in the Mailbox Store), and retries to send them or returns
NDR to the sender. |
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