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I've got an SMTP server running on IIS Windows Server 2003 that sends mail out via another relay - sometimes that mail doesn't arrive.

Where can I find out why mail isn't sent past the relay?

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Which software package works as SMTP server on your Windows Server? (IIS is not an SMTP server, and it cannot run one.) – Moritz Both Sep 7 '11 at 15:27
@Moritz er.. yes it can -… – Sam Sep 7 '11 at 15:45
Thanks for the hint, Sam. I was unaware of that. In this case, I cannot contribute here. – Moritz Both Sep 7 '11 at 19:33
up vote 1 down vote accepted

"Past the relay" is not possible. Your IIS should confirm successful delivery to the relay or give an error on unsuccessful delivery. It is a bad idea to ignore the return code of this delivery.

Everything else behind the relay is out of your responsibility and out of your scope. It cannot be traced without the help of the admins of the relay and the help of the admin of the servers behind that relay.

To make sure you get informed by servers behind the relay when they are unable to deliver a mail you must ensure that the Reply-To:-Header, the From:-Header and/or the envelope-sender are valid email addresses and not like

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Okay. I'm sending out via ASP.NET and Classic ASP, so I'll have to look up the relevant settings. Thanks! – Caveatrob Sep 7 '11 at 22:43

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