MX Record Error - Mail Returned

I'm running into a little problem with my mail server. A client of mine can't send e-mail to a certain domain. It is returning the e-mail with the following error:

> > Delivery failed
> > 550-Verification failed for <e-mail address omitted>
> > 550-It appears that the DNS operator for domain omitted
> > 550-has installed an invalid MX record with an IP address
> > 550-instead of a domain name on the right hand side.
> > 550 Sender verify failed

I checked the MX record, and it was pointing to the mail server's IP Address. I changed it to point to the mail server's domain name instead. But when the client trys to send e-mail it still returns this error.

Is it possible that the receiving mail server has the old MX record's information cached? Is that usual? Any ideas how I can look into this error more, and see if I am missing something? Thanks in advance for any help.

I haven't seen anyone answer this so I am going to take a stab at it. It looks to me like the email server that you are connecting to is trying to do some kind of a reverse lookup or verification on your email address and is unsuccessful. I would recommend doing a whois for that servers address, taking the DNS informatoin from there, connect to their DNS server via nslookup, set your query to mx, and then look yourself up and see if you get an MX record back and that it is right (or set the query to all and make sure that it is right and they are getting a reverse lookup).

You didn't really indicate what direction you are having the problem with either, whether it was a problem with someone sending mail to your server or your problem sending mail to another server. So it is kind of hard to say with certainty where the problem lays, but I would start out checking DNS on both ends to make sure they properly resolve. If you need help doing what I mentioned above, PM me and we can talk about it there.

/nebulus

nebulus200, thank you for the suggestions. The error occurs when sending mail from my server to theirs. The e-mail is returned with that error. I checked out my DNS settings, and everything seems to be correct now, though the MX was pointing to an IP address originally. That is why I was wondering if their mail server may have had my mail servers old MX record cached in some way.

Could you go into more detail about how I can look myself up from their server using nslookup? I know how to look up there server information from my location, but I don't know how I could look up myself from them. Thanks.

Ensure that you have increased the serial number on the DNS zone, otherwise it may cache the entry forever.

This is a common mistake I've seen it before. Trouble is, some MTAs deliver mail with these broken MX records, which doesn't help people to configure them correctly.

Excellent point Slarty, I would like to mention, even though I have pm'd you with details, is please mention if you find the solution to the problem and what fixed it, especially if it wasn't what we mentioned, because I personally always enjoy learning new things and would like to learn a little more.

/nebulus

I've run across this only one time and it had to do with the firewall I was using and W2K. Specific firewall was Watchguard, some email would work others not even befor the W2K upgrade they did and it turned out I needed to proxy the email connects through the firewall on this combo. If there has been an upgrade on anything mial server or firewall check to make sure a proxy is not needed.