Home » Posts tagged 'NDR'
Tag Archives: NDR
This week I was called up by a former employer to come in and check out their Exchange 2003 server. They had been having problems with a few domains rejecting their e-mails for the past few weeks and what made it a big problem was that one of the domains was their largest customer. Not a good scene there. They were receiving several NDRs of which the most prominent was something like <mail.domain.com #5.7.1 smtp;554 5.7.1 firstname.lastname@example.org: Relay access denied> as well as some from another domain referencing authentication. I don’t have it in front of me so unfortunately I can not quote it. Doing some research I found that there was not much information on troubleshooting this and related problems even though there were lots of people asking. So I am putting together a quick little guide based off how I came to a resolution. Your results may vary.
E-mail problems generally break down to two types. Either you are having DNS problems, or you have a misconfigured server. First thing to do of course is to narrow down whether the problem is on your end or if it is on their end. Generally one would suspect the problem to be on their end when it is only a couple of domains giving you a problem, but that isn’t always the case. Easiest way to narrow it down is to break out the command line utilities. First verify that you are using the same DNS as your Exchange server. This is very important otherwise it may skew the results of your test. Next off you need to find your problem domain’s mail server. You have several options for this. The lazy way is to go to MX Lookup Tool and put in the domain. It should return the mail servers for that domain. You’d be best off going with the first one. The site also has some interesting diagnostic utilities that you could run against your mail server as well as their own. The non-lazy method, and one you should learn anyhow, is using nslookup. Pop open your command line and run nslookup. Here’s the steps to go through to get the MX records for their domain.
> set type=mx
problemdomain.com mail exchanger = 10 mail.problemdomain.com.
Authoritative answers can be found from:
There’s the mail server that you are working with. Sometimes there are more than one so start from the top and work your way down. Next up is we’re going to send an e-mail the archaic way, through telnet. Note that not all SMTP servers respond with the exact same banners and responses, though the codes should generally line-up. Open up a telnet prompt.
telnet> open mail.problemdomain.com 25
Connected to mail.problemdomain.com.
Escape character is ‘^]’.
220 mail.yourdomain.com ESMTP Sendmail 8.9.3/8.9.3; Tue, 27 Aug 2002 16:20:32 -0500
At this point you need to identify yourself as your mail server.
250-mail.problemdomain.com mail.mydomain.com [126.96.36.199] pleased to meet you
Now it is time to identify who you are.
Next is who you are e-mailing.
rcpt to:email@example.com notify=success,failure
If you get a 250 Ok at this point then telnet will most likely complete successfully. Also if you get a 510 Bad user that is ok as well, since you are not getting a relay access denied which is what your Exchange server is getting when it communicates with this server. This means their server has passed the telnet test so you need to start digging into your server to fix the problem. If you get a 550 Relaying denied error though that is different. Time to modify the test. Change your DNS server to a known good external DNS server that resolves both forward AND reverse look-ups correctly. If it does not resolve reverse look-ups it is no good to you. Switch your machine over to that DNS server and run through the test against. If it is successful then you’ve tracked the problem down. Time to look at what DNS servers your Exchange is using. Or it may also be time to check on your own internal DNS servers, as they may not be resolving addresses correctly. If, though, you receive a failure from your telnet test with a known good set of DNS servers then things may be a bit more complex. Do a look up on whether your domain is in any Realtime Blackhole Lists (RBL). If it is then definitely check to see if you are running an open relay and get yourself off those lists fast. If not then you had best get on the phone with the admin over at problemdomain.com to find out if your domain has been blacklisted. Finally another thing to verify is your SPF record on your external facing DNS. Here is a site to help craft an SPF record.
For completeness here are the rest of the directions to finish sending your e-mail through telnet:
354 End data with <CR><LF>.<CR><LF>
Connection closed by foreign host.
In case you were wondering at the client’s site it turned out the Exchange server was set up to use an external DNS server that was not resolving reverse look-ups. This was causing a few sites to return the 554 relay access denied NDRs.
Some other tools that may be helpful in diagnosing your problem: