I had the same problem. I realized I wasn't sending it to a valid email address. When the registration form came up I entered in a valid email address. then it worked
Webcofig:
<
system.net><mailSettings>
<smtp>
<network host="mail.xxx.com" userName=xxx@bluebottle.com password="xxx"/>
</smtp>
</mailSettings>
</system.net>
<system.web>
Code behind:
<MailDefinition From="xxx@bluebottle.com" BodyFileName="~/Register.txt" Subject="confirmation"></MailDefinition>Then:
In the registration form put in a valid email address