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We've run across something strange.

We're running ModusMail 1.4
We run DNS in our network and assigned our mail server a priority of 10
We use our backbone provider as secondary DNS *AND* secondary mail
server (smart host?).  We have the smart host entered into our DNS with
a priority of 20.  The backbone provider's DNS (secondary) matches the
above.

We began getting contacts from people (non-customers) indicating that
when they attempted to send a message to our domains, they would get a
"user unknown error".  We verified the ModusMail server was alive and
the account was ok.  We also found that only specific systems were not
able to deliver these messages to our domain while other systems
continued to deliver messages to the same accounts with no problem.
Ultimately, we learned that the back up email server had gone off-line
and removing the back-up mail server from our DNS made the problem go
away.

The back-up mail server was repaired, returned to service and we added
it back into DNS. Everything runs well for about a week.

Today, I start getting contacts that external users are again getting
error messages, but this time they are "mailbox unavailable (SMTP Error
Code 550)".  Guess what - the back-up mail server had died again.
Removing it from DNS corrected the problem.

Why would a failure of a backup mail server that is listed in both DNS
with a higher priority than the primary email server cause a "user
unknown / mailbox unavailable error"?

I can't understand why a failure in a back-up server would be service
effecting ???

Bill Rakowitz
YK Communications




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