This error may occur when you are working with .net 1.1 means when using the namespace Sytstem.Web.Mail. If we want to send email to multiple or more than one person usually we use like:
This will work in the namespace Sytstem.Web.Mail because here we use semicolon to split or separate email ID's.
But the ERROR: "Specified string is not in the form required for an e-mail address" will appear when we use semicolon as a separator in .net 2.0. Because the namespace System.Net.Mail in .net 2.0 framework will use comma as the email separator instead of semicolon. So we will use like below:
Hope now your problem has been resolved.