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External Email Delivery Failure/Delay

Posted on 2010-01-05
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Last Modified: 2012-05-08
I am able to receive emails from external mail servers with no problems. However, emails from users on my network are not being accepted by the destination mail servers. I am able to look at my firewall logs and network captures of the traffic from my mail server. Traffic is is leaving my mail server and connecting to the external mail servers on port 25. In the packet capture the my mail server is only making a SYN connection to the external mail servers and they are not responding.

Its as if the external mail servers don't want to talk to my exchange server. I checked to see if my IP address was blacklisted and it is not.

Could it be a configuration error with the exchange server?
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Question by:CompL-3
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by:MrMintanet
ID: 26181741
Can you send mail to mass mail servers like gmail or yahoo?

Try sending to computeridioth@gmail.com
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by:giltjr
ID: 26181800
In the firewall log does it show the SYN going out?
Does it show the response coming back in?

What firewall are you running?  
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Accepted Solution

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databit54 earned 1000 total points
ID: 26181810
Goto http://www.mxtoolbox.com/ and run a full diag against your exchange server. See what the results are.
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Author Comment

by:CompL-3
ID: 26181972
The firewall just shows connections. I set up a packet sniffer on the WAN port of the firewall and saw that only SYN packets were getting through. There were no SYN_ACKs coming back from the mail servers.

I ran a full diagnositic using the www.mxtoolbox.com and it responded with everything being ok except the following:

"Warning - Reverse DNS does not match SMTP Banner" I am not sure what that means.


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Assisted Solution

by:MrMintanet
MrMintanet earned 1000 total points
ID: 26182020
Reverse DNS has to be set by the company responsible for the IP address and this is usually your ISP. The reverse DNS should ideally match the MX record, and what your server has on the SMTP banner when connecting. However some companies will simply accept your email because you have a reverse DNS entry, not worrying that it doesn't match what is in the SMTP banner.
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by:giltjr
ID: 26182152
What kind of ISP account is this?  Business or residential?

Some ISP's block port 25 on residential accounts because they don't want you spamming.  They force you to send e-mail through their SMTP servers.
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Author Comment

by:CompL-3
ID: 26182185
We are using a business account, but I will check on that. I will also check the reverse DNS as I think that may be the issue since we just changed ISP's.

Thanks!
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by:MrMintanet
ID: 26182188
;)
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by:giltjr
ID: 26182230
I don't think the reverse DNS would be the issue.  The reason for that is that the remote side would have no clue what your SMTP server is sending on the HELO/EHLO until after a the TCP connection is made.

Since the TCP connection is never being established there is no way for them to know that there is a mis-match.
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