Customer sent an email through SmarterMail vis SMTP, and got the following message back from the recipient:

Reason: Remote host said: 421 invalid sender domain, possibly misconfigured

I can't find any information relating to the message on the web.

We have domain keys setup, SPF records, and whatnot, so I can't see why its misconfigured. Never had this issue before.

The only thing I can think of, is that all our clients domains send from the same IP address (much like hostmysite's shared email servers do), and that information is in their SPF records. My guess is they are trying to match the domain to the reverse DNS of the SMTP server and they aren't matching.

A 4yz type error denotes a temporary delivery failure. This error indicates to your mail server, that it should attempt to resend the email later.

Reference: http://tools.ietf.org/html/rfc5321#page-48

With SmarterMail this is typically an indication that you should increase your retry timeout settings.

Refrence: http://www.smartertools.com/forums/p/16512/41113.aspx#41113

Nate's right - a 421 error is by definition a temporary error and your server should 'ignore' that and keep attempting to resend the message.

If you want to doublecheck I'd advise using a 3rd party 'test an email address' tool, such as the one found here: http://www.hq42.net/net_tools/test_email_addr.php

I think Nate also has one on his website as well.

Thanks guys. I'll wait and see what happens. I'll give the email utility a try that Jamie Suggested.

@Jamie, that URL doesn't seem to be working. What checks does it do? I have an Address check tool, and a Relay Check tool. Unforuntately the latter is whitescreening at the moment, and I haven't had a chance to investigate why (it works in dev ) ...

<edit>Found out the tools were broken due to a bug in an older version of ZF which did not contain the proper validation classes!</edit>