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I have recently setup a new SMTP server on my new webserver (win 2008).

The server sends mail from a number of domains hosted on the website, everything seems to work OK but I am getting a number of undeliverable reports which state '421 unable to verify sender domain'

What does this mean and what should I do about it?

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migrated from stackoverflow.com Nov 25 '09 at 4:46

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3 Answers 3

Things I would make sure of first:

  1. Make sure you have a valid rDNS (PTR) record for the IP address of the server. A lot of anti-spam tools that filter or reject email perform a reverse lookup to verify the hostname of the sender matches where it originated from.
  2. Make sure your HLO/EHLO command correctly matches the rDNS hostname.
  3. Setting up an SPF record is also not a bad idea if you don't already have one: http://en.wikipedia.org/wiki/Sender_Policy_Framework
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This error has a specific meaning in the specifications, but in today's spam-filled world, it often means that your mail was flagged as unacceptable.

Try sending mail from another host to the same place, with a popular hosted service (like gmail). If it gets through (and other places receive your mail), you were probably filtered.

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Could it be something with an FQDN and/or DNS - check the domains that work and their DNS with the domains that don't.

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