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DKNUCKLES
Asked: 2012-01-14 08:23:37 +0800 CST

Some senders from external domains receiving "User Unknown" Errors

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As far as I can tell, this question doesn't already exist here but if it does then I apologize.

As of January 1st our company name has changed and, as such, our e-mail addresses have changed. The switch has gone off well with the exception of a few weird "5.1.1 User Unknown" errors that some external senders are getting when they attempt to send e-mails to us.

I've spoken to an external consultant to get their take on it and they seem to think the recipients have stale DNS records. The MX records to our server has existed on our external DNS for over a month - the new domain was set as our "Primary" domain on January 1st.

I've managed to get a couple of the error reports from the recipients and they look like this. Any help would be appreciated with this matter

The following recipient(s) cannot be reached:
      'User Name' on 13/01/2012 11:07 AM
            550 5.1.1 <[email protected]>... User unknown

EDIT : The domain listed above is the "new domain". E-mails sent to the old domain still send without issue. I can also verify that the policy has been set to that user and that the new e-mail address exists within their "E-Mail Addresses" tab.

EDIT 2 : The receiving mail server is running VamSoft ORF for anti-spam. Neither Vamsoft ORF logs or exchange logs show any sign of the e-mails getting to our domain, however when they use the old domain they go through without issue.

domain-name-system smtp exchange-2010

2 Answers

  1. The key to troubleshooting this one is going to be getting somebody to look at the logs on the outgoing SMTP transaction at the sender's server. You can try to correlate their failures with your own message tracking logs, SMTP protocol logs, or traffic sniffing, but the best data is going to come from the sender's own SMTP server. If the sender has an IT staff I'd recommend working to get in touch with them. If they don't, then try to get the sender to provide you with the non-delivery reports complete with full transport headers.

    You need to know what SMTP server the sender is actually trying to perform delivery to and, from there, you should be able to track down the source of the problem. If they're talking to a server you control then you should be able to get a log of the SMTP conversation, possibly all the way down to a traffic sniff. If they're not talking to a server you own then you need to figure out why they aren't (i.e. stale DNS cache).

    DNS lookups cached for wholly inappropriate times aren't completely unheard of, but they're not common either. If you know who the sender is using for an upstream DNS server you might try doing some lookups for MX records for your domain against that DNS server to see what you get back.

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  2. I've spoken to an external consultant to get their take on it and they seem to think the recipients have stale DNS records

    Send this "consultant" to ass! He generates illiterate nonsense! I SMTP-session installed with correct collector (assigned as MX for domain and domain handled on server) and only some e-mails doesn't map to existing destination problem is your local rewiting rules|MDA in Exchange or (probably existing) border-MTA - can't say more without exploration in deep delivery-path for your mail-subsystem.

    Sadly, I know nothing about Exchange's MTA (except well-known in postmaster's world denominator "Bullshit!"), but, as long-time postmaster, can see:

    • you have to identify, which your server really generates bounce (in case of chained processing "border - internal")
    • better will be to find all e-mails (local users), which incoming mail are bounced
    • debug (monitor) incoming SNTP-sessions for such users in real-time (if trouble-generator have ability to debug) OR compare "bad" and "good" users and search difference in definitions

    Added after discussion with Evan

    You have to idenify, that undelivered mail is your problem on your server. Ask any owner of bounce-mail forward ("MIME-forward", with full original message as attachment) responce to you and check headers of bounce (Received) in order to confirm (or reject) involvement of your MTA as bouncer

    From my side I can offer (some amount of) my time, attention, mind, hands and my smtp-emitter for test-runs of incoming SMTP

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