めも:'System.Web.Mail.MailMessage' じゃだめなのか。。。
Warning 15 'System.Web.Mail.MailMessage' is obsolete: 'This class is deprecated. Please use the System.Net.Mail classes instead.
Warning 15 'System.Web.Mail.MailMessage' is obsolete: 'This class is deprecated. Please use the System.Net.Mail classes instead.