Troubleshooting 421 4.0.0 error


I hope this may be useful to someone. I have just spent a couple of hours troubleshooting the above error message which was occuring when we sent mail to our company banker.

I appears that others are having similar problems see the thread here:

Anyhow, I thought I would list my troubleshooting steps.

On being told that people couldn’t send mail to the recipient, I proceded as follows:

First I looked at the queues on the Exchange server and located the mail. It was set to retry and had been that way for a day or so.

I next checked the error message presented in the queue viewer which was as below:

“The connection was dropped by the remote host”

I then restarted the SMTP service to see if that would get things moving. As expected, it didn’t!

Having done this I then used nslookup to lookup the MX records for the problem domain. I did this by first setting nslookup to search for MX records using the command:

Set type=mx

then entering the domain.

Once I had the mail servers for the domain I new DNS was working so I then moved on and checked the SMTP logs by going to:

Start/Run/ and typing logfiles.

Then I located the SMTPSVC1 folder and copied the latest log to my PC

I opened the log using Excel and converted the log to columns using?a space delimiter in the wizard.

I then filtered the log to only show the domain in question. This showed that many connection attempts were made but all failed with a “421.4.0.0 Server error” message.

Having established this I tried to telnet to the mail server from my Exchange server and then from another box. Both connection were refused in the same way.

Given the immediacy of the connection drop either we were on a block list or connection filter, or perhaps we were missing a PTR record.

My next step is to get the relevant people to setup the PTR records which is not currently there. I believe it is likely that this will then allow mail to flow.

I have also changed the way the SMTP server announces itself in the advanced section of the delivery tab on the SMTP Virtual Server so that it now announces itself using the same address as the MX record.

I will report back soon with progress.