Is your SMTP server configured so that it will send mail out of your internal network? Can other applications send email using this server? What error messages do you receive? Also, take a look in the Event Log Viewer (on the Admin tab) to see if there are other error messages or more details.

Could you please post the entire error message. I'd like to read the part that tells you why the mailbox is unavailable. If it is because of a relaying issue, you should be able to show that message to your IT department and they should be able to fix it.

In other words, this appears NOT to be an issue with DotNetNuke, but it appears to be an issue with the SMTP server, it's configuration, or how you are connecting to it. Your IT folks ought to be able to help with that.

Message: System.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable. The server response was: 5.7.1 Unable to relay 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, String[] Attachment, String SMTPServer, String SMTPAuthentication, String SMTPUsername, String SMTPPassword, Boolean SMTPEnableSSL)

The key phrase in this error message is "unable to relay." The SMPT server is configured so that it won't "relay" mail for any user. You'll need to take this to your IT department, tell them that:

- your DotNetNuke installation needs to use the SMPT server,
-as you are using it now, the SMPT server is refusing to cooperate because it doesn't recognize your site as a proper user, and
-they need to fix it!

It's THEIR problem to fix! (unless you are your own IT department )