I can confirm that DNN 4.02 (and now 4.03) has problems with sending e-mail from some modules.  For example, when I attempt to add/remove a user from a security role, I have to turn OFF the e-mail notification option.  If I don't, I get this error:

Error: User Roles is currently unavailable.
DotNetNuke.Services.Exceptions.ModuleLoadException: The specified string is not in the form required for a subject. ---> System.ArgumentException: The specified string is not in the form required for a subject. at System.Net.Mail.Message.set_Subject(String value) at System.Net.Mail.MailMessage.set_Subject(String value) at DotNetNuke.Services.Mail.Mail.SendMail(String MailFrom, String MailTo, String Cc, String Bcc, MailPriority Priority, String Subject, MailFormat BodyFormat, Encoding BodyEncoding, String Body, String Attachment, String SMTPServer, String SMTPAuthentication, String SMTPUsername, String SMTPPassword) at DotNetNuke.Services.Mail.Mail.SendMail(String MailFrom, String MailTo, String Bcc, String Subject, String Body, String Attachment, String BodyType, String SMTPServer, String SMTPAuthentication, String SMTPUsername, String SMTPPassword) at DotNetNuke.Modules.Admin.Security.SecurityRoles.SendNotification(Int32 UserId, Int32 RoleId, String Action) at DotNetNuke.Modules.Admin.Security.SecurityRoles.grdUserRoles_Delete(Object sender, DataGridCommandEventArgs e) --- End of inner exception stack trace ---

After reviewing this thread, I'm beginning to believe that this is a recurring problem with sites "upgraded" to DNN 4.0.x.  I do NOT have this problem with child portals, so I'm thinking that it is an upgrade issue, and not a 4.0.x issue.