Deberías mirar en el visor de eventos para averiguar más detalles sobre el error. Probablemente te dirá el motivo por el que no puede enviar el email y así podrás solucionarlo.

Salu2,
Francisco

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Francisco Pérez Andrés
DOTWARE · tecnología a punto
www.dotware.es
www.dnnole.com

Estiamdo, aqui te envio el log:

AssemblyVersion: 5.6.2
PortalID: 0
PortalName: Virtual CQ, Soluciones Informáticas
UserID: -1
UserName:
ActiveTabID: 64
ActiveTabName: Contacto
RawURL: /Contacto.aspx
AbsoluteURL: /Default.aspx
AbsoluteURLReferrer: http://www.virtualcq.com/Contacto.aspx
UserAgent: Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/50.0.2661.102 Safari/537.36
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: 0cd827ce-86b7-43f9-93e0-fb1c0671124b
InnerException: Mailbox unavailable. The server response was: From address must match authenticated address
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.Net.Mail.MailCommand.CheckResponse
StackTrace:
Message: System.Net.Mail.SmtpException: Mailbox unavailable. The server response was: From address must match authenticated address at System.Net.Mail.MailCommand.CheckResponse(SmtpStatusCode statusCode, String response) at System.Net.Mail.MailCommand.Send(SmtpConnection conn, Byte[] command, String from) at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception) at System.Net.Mail.SmtpClient.Send(MailMessage message) at DotNetNuke.Services.Mail.Mail.SendMail(String MailFrom, String MailTo, String Cc, String Bcc, String ReplyTo, MailPriority Priority, String Subject, MailFormat BodyFormat, Encoding BodyEncoding, String Body, List`1 Attachments, String SMTPServer, String SMTPAuthentication, String SMTPUsername, String SMTPPassword, Boolean SMTPEnableSSL)
Source:
Nombre del Servidor: IIS3003

saludos