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Exchange 5.5 - Mail Delivery Problem

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10 replies

by dman76

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Approximately a few days ago, users sending mail to some sites started getting their mail kicked back to them. Looking at the SMTP logging in Event Viewer, we were receiving an error 451. Looking at the queue properties for the non-deliverable mail,the error that we were receiving was using generic terms:

[451 4.1.8 ... Domain of sender address sender@sendersdomain.com does not resolve.]

Doing an nslookup for the mx record to the recipient domains was successful, and an nslookup using the recipients server to do a lookup on our mx record was successful, ping and tracert were also successful, and telnet sessions to the remote mail servers also worked, so there is definately communication back and forth between our respective email servers. Our Exchange server is on NT 4.0 with service pack 6, and Exchange has service pack 4. If anyone has any idea what the cause of this error is, could you please help! I did find the same error reported by an nscloss in this Q&A, but no answer to the problem. I have been searching everywhere!

10 replies

by wineguy30

I'm sure you've thought of this, but is there any chance that your Exchange server is acting as on open relay? Many domains will not accept email from your domain if anyone can use to your server to send spam out. Just a thought....

by dman76

No, we are not acting as a relay, that was a problem that we had in the past but has been remedied some time ago. Thank you for your thought.

by estebandelatorre

You ahve experienced a DNS error. When the problem happens, the dns were unable to resolve domain names to mx ip address of your receivers.
Now is your email working good?.
Follow this rules.
Allways have at least 3 DNS servers configured into your email internet agent.
Try that those DNS are on different networks and don?t belog to the same provider. Providers won't tell you that theirs DNS is down unless you have enoght screen captures about ie browsing and Nslookup's.
Have a person monitoring counters of i/o mail and keep a network console monitoring the queue "Problem to send"
Depending on your current email configuration (this is regarding to the internet agent) check it's configuration, usually 5 hours is the default to keep on sending a message until bounce it, change it to 5 days!

by brasslet

A couple of possibilities come to mind:

Even though you can do an nslookup for your domain, mx record etc. that doesn't mean reverse lookup is configured. Some mail servers will require this to accept mail. You can check to see if this is configured by contacting your DNS provider (ISP), or go to www.network-tools.com for a quick check.

Not as likely: you said you were an open relay but fixed it. Did you also go to rbs.org and let them know your relay was fixed?

by dman76

The reverse lookup was one of the first things we checked, we look after our own dns, and it is configured correctly.
RBS.org is aware that our relay is fixed, this was a long time ago.. (8-12 months). Thank you for your input, but still looking! 8)

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