Illegal Semicolon

Illegal Semicolon

Trying to send an email to several people.  I inputted a few email addresses in the TO: field, separated by a semicolon.  But before I can send it, I get a red window saying that the email has an " Illegal semicolon, not in group" followed by the lists of names.

The message is halted before sending.  Any idea what the problem is?

Thanks.