Multiple Recipients in Outlook

I am using MS Outlook XP 2002 (not Express).
I am also using Norton Anvirus 2003.

I recently sent an party invitation email to 11 people in my address book, but the message would not empty from the Outbox. I ended up having to send the message to each person individually. I now need to send a message to about 50 people in my address book, and I have encountered the same problem. I don’t want to send 50 individual emails out this time!

I have sent a test message to several addresses (5)successfully. I have included a non-existent address, and the email was correctly delivered to the real addresses.

Is there some function in Outlook which restricts the number of recipients for an outgoing email? Any other ideas?

Thanks