You need to contact your hosting provider as they will have some specific settings that make it possible to send mail out from the hosted server.

Sometimes it's 127.0.0.1, sometimes localhost, sometimes another IP, sometimes they have a full mail server address which handles the sending of mail between servers.

Nina Meiers

This is really frustrating:
If I enter localhost, 127.0.0.1 or my Internet provider's SMTP server, everything seems to work (at least e-mails are sent and received successfuly when I click the Test link on the Host Settings page). However, when I turn the e-mail notification on in the Forum module, an error occurs any time when a notification should be sent. I tried another forum module and behavior is the same - some error again. The action which the user should be notified about (e.g. adding a post in a forum) is always performed, but the notification crashes.

The Test link sends e-mails properly, but modules cannot:
the error always occurs in System.Net.Mail.MailMessage..ctor(String from, String to), where String to is empty. All registered users must have entered their e-mail addresses, so I really don't know why it has no value. The first DNN function in the chain is DotNetNuke.Common.Globals.SendNotification(String MailFrom, String MailTo, String Bcc, String Subject, String Body, String Attachment, String BodyType).

Please help someone or I will hang myself :).