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SMTP connection timeouts on Exchange 2003 server.

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Last Modified: 2012-05-06
We are having a problem with our Exchange 2003 server where sometimes e-mails will not be received.  The sender does not get any bounced messages, the message just disappears.  We have disabled or removed antispam, antivirus, and turned off all filtering in Exchange.  The SMTP logfile still shows:

0 EHLO 250 0 SMTP -
0 MAIL 250 0 SMTP -
0 RCPT 250 0 SMTP -
0 TIMEOUT 121 1075732556 SMTP -

For many of the connections.  Any ideas on this?

Thank you
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Expert of the Quarter 2009
Expert of the Year 2009

Commented:
What is between the server and the internet? Firewall of some kind?

-M
which one of the following do you mean?
1. Internal users cannot receive emails coming from outside
2. Internal users cannot receive emails coming from other internal users
3. Outside reciepient cannot receive email from internal users
OAC TechnologyProfessional Nerds

Author

Commented:
Internal users cannot receive SOME e-mail from outside.  It seems random.  Outside recipients will always receive e-mail from the internal users.

There is a DSL modem/firewall between the server and the internet.
check to see if you have multiple MX records for your domain
When you say it seems random, are you saying that specific outside users may get through one time but not another? (rather than, say, the same outside people being blocked consistently, but it being random which users those are)
OAC TechnologyProfessional Nerds

Author

Commented:
Only one mx record is set
OAC TechnologyProfessional Nerds

Author

Commented:
It seems to be they may get through one time but not another.  Although, the timeouts seem to be happening for comcast.net's connections pretty constantly.
Expert of the Quarter 2009
Expert of the Year 2009

Commented:
Really need to be more precise with what is between the internet and the server, as some firewalls have SMTP scanning features which can cause this kind of problem.

-M
OAC TechnologyProfessional Nerds

Author

Commented:
It's an Actiontek M1000 from Qwest.  The firewall is turned off on this modem.  That's the only thing between the internet and the server.
Press2EscSystems Integrator

Commented:
"Internal users cannot receive SOME e-mail from outside.  It seems random.  Outside recipients will always receive e-mail from the internal users."
Seems to be more a POP issue than SMTP...  Also, the IPs of the undelivered email may be being blocked or blacklisted - have your checked??  Any specific NDR or associated email header would be helpful....  P2E
Professional Nerds
Commented:
It turns out that I needed to change the MTU settings on the server to 1400.  Apparently since there was no router between the modem and the server, the server's MTU settings were too high for that DSL connection.  Once this was done, the connections all went through successfully and instantly.

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Press2EscSystems Integrator

Commented:
Excellent DD.....  I am curious, what was the failing MTU value - 1500??
OAC TechnologyProfessional Nerds

Author

Commented:
Yes, it was set to 1500 before
Press2EscSystems Integrator

Commented:
I suspected you'd say 1500 because of the DSL overhead, 1492 is the highest packet size DSL is able to handle...
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