The MailDefinition.BodyFileName property specifies the location of the text file to use for the body of the e-mail message.

The MailDefinition.CreateMailMessage(string, IDictionary, System.Web.UI.Control) method uses its owner parameter to map the MailDefinition.BodyFileName property to a path on the server. If the owner parameter points to a control on an .aspx page or to the page itself, the MailDefinition.BodyFileName property points to a text file in the same directory as the .aspx page. For example, if the .aspx page is located at C:\inetpub\wwwroot\default.aspx, the System.Web.UI.WebControls.MailDefinition control looks for the text file in the directory C:\inetpub\wwwroot.

If the System.Web.UI.WebControls.MailDefinition class is used in a user control contained in an .ascx file, the MailDefinition.BodyFileName property points to a text file located in the same directory as the user control. For example, if the .aspx page is located at C:\inetpub\wwwroot\default.aspx and the user control is at C:\inetpub\wwwroot\controls\control.aspx, the System.Web.UI.WebControls.MailDefinition object looks for the text file in the directory C:\inetpub\wwwroot\controls.