This is a bug in the documentation for the .Net 2.0 version. The msdn documentation for the "to" parameter says:
1. The e-mail addresses to add to the MailAddressCollection. Multiple e-mail addresses must be separated with a comma character (",").
2. "If multiple e-mail addresses separated with a semicolon character (";") are passed in the addresses parameter. a FormatException exception is raised."
The .Net 1.1 version uses System.Web.Mail where the receipients have to be added semicolon separated. .Net 2.0 uses System.Net.Mail where the receipients have to be added comma separeted. (Why in the hell do they change this behaviour from semicolon to comma where all toher standards are using semicolon???)