when a new user try to register an error is raised

ModuleId: -1
ModuleDefId: -1
FriendlyName: Register
ModuleControlSource: Admin/Security/Register.ascx
AssemblyVersion: 04.00.03
Method: System.Net.Mail.MailMessage..ctor
FileName:
FileLineNumber: 0
FileColumnNumber: 0
PortalID: 0
PortalName: lo2ta
UserID: -1
UserName:
ActiveTabID: 36
ActiveTabName: Home
AbsoluteURL: /Default.aspx
AbsoluteURLReferrer: http://www.mcabayarea.org/home.aspx?ctl=Register
ExceptionGUID: e5daa014-8cd1-4a28-a4e2-ac6290d7daad
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
InnerException: Value cannot be null. Parameter name: from
Message: DotNetNuke.Services.Exceptions.ModuleLoadException: Value cannot be null. Parameter name: from ---> System.ArgumentNullException: Value cannot be null. Parameter name: from at System.Net.Mail.MailMessage..ctor(String from, String to) 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.Register.cmdRegister_Click(Object sender, EventArgs E) --- End of inner exception stack trace ---
StackTrace:
Source:
Server Name: WEBC16

i dont know what does it mean