If you have recently moved from the System.Web.Mail (.net 1.1) to System.Net.Mail (.net 2.0), you might get this error message when sending in a string of mail recipients, and when debugging, the emails look good to you! The reason is the delimiter. I cannot imagine what twist of fate movitivated the Microsoft Class gods to switch a standard on us, maybe it was just to keep us on our toes, but they did. We used to delimit our email addresses (multiple to's, cc's, bcc's, etc) with semi-colons (;) as we do in outlook, but that no longer works, now the class only accepts the comma (,) as the address delimiter. Go figure.
So, if you have a large established code base, and you want to upgrade your mail class, just one class that you use for ALL your applications, what do you do to minimize your rework? My advice is to use the string replace function and just replace every instance of semicolon with a comma. That will handle the old code passing in semicolons as well as the new code passing in commas. Then you have one place to make the update and you can leave your legacy code alone.
If you are NOT using a single mail handler class across your applications, then you're working too hard!