The error message usually tells you which address is bad, but the others will usually go through. With my service, if I try to send more than 25 bcc's it will stop them (which is a problem sending notices to our band email list). There are ways around this if you contact your ISP or spend hours reading their tech pages.

Check the error message again and see if it lists specific addresses that didn't work. Check the original email to be sure each address was separated by a semicolon. Limit the number to 25 at a time for each send.

Let us know how you make out. Hey, and I'm in your address book and I didn't get anything from you!