Hi there,
I have had this a few times as well, and usually it was related to the "relaying" feature of IIS. Is your SMTP server relaying to another server? That is, do you forward the messages by using a Smart Host in the IIS / SMTP settings?
If that's the case, the server you forward to may have stopped accepting relayed message, a common practice to stop unwanted spammers from abusing your server. Find out what security mechanism the server uses, and whether you're blocked or not.
HtH
Imar
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Imar Spaanjaars
Everyone is unique, except for me.
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