I set up an Exchange distribution list and added about 25 people to the list. I then created an End User in Service Desk with exactly the same name and set the e-mail address to the e-mail address of the distribution list with External set. I then set the user on a reminder note control to the distribution list user. Our SMTP server is a secure connection but since I can send out other e-mail and pages using SMTP I can't imagine why the distribution list is not working. In my event log I get an error that the user can't be authenticated. This leads me to believe that because it is a distribution list it does not have an associated user account and thus fails. I got around the issue by using a Service Desk Support Group but I would rather the owner of the distribution list make changes and not me.
Anyone have any experience with this and know how to fix it?
It has been a while since I have set this up but it is likely down to the configuration on the distribution list. Try and send an email to the list using its full SMTP email address (ie. firstname.lastname@example.org) from both an address on the Exchange server and an address external to it and see what happens in each case.
Also in Service Desk in your outbound mail settings have you specified a username and password? Those are optional but in most environments they are needed for things like this.