"Message delivery to the remote domain 'yahoo.com.tw' failed for the following reason: The connection was dropped by the remote host."
Even after restarting the SMTP service, these messages with different domains are coming. Also, the domains are getting resolved when I try NSLookup from that same IIS server where SMTP service is running. Some domains are not valid, while some are valid, and the mail should go to those domains. Can you please help?
Unfortunately, the error message in your log isn't specific enough to identify the problem you're having. If you're using IIS' SMTP service, add your address to the Send Copy Of Non-Delivery Report To field. Hopefully, that will have more useful information.
If the problem is intermittent, and mail does eventually get delivered to the destination domains, then I wouldn't worry about it -- it's normal for some SMTP connections to fail. SMTP is designed to try again later, so everything should be fine. If the problems occur consistently with some domains, though, it's possible that there's an incompatibility (unlikely if you're using popular software that has been updated), or that messages from your computer are being actively blocked. This can happen if you are added to a spammers' list, or if your computer is running an "open relay."
Alternatively, if you're familiar with the SMTP protocol, you could connect directly to the problematic destination SMTP servers using Telnet and attempt to send a message manually. The server is likely to provide you with a more specific error message that will be useful in identifying the problem.
This was first published in October 2003