How do I add a guest mail user to adress lists or create a mail contact using the same email address

To be honest, it's a bit of a mess as you can actually have the same address added twice:

[10:36:59][Login script]# Get-Recipient tempxxx@abv.bg | Ft Name,RecipientType*

Name                RecipientType RecipientTypeDetails
----                ------------- --------------------
temp                MailContact   MailContact
tempxxx_abv.bg#EXT# MailUser      GuestMailUser

The "trick" is to have the contact first, then provision the Guest account. Doesn't really make much sense to me, but TonyRedmond seems to see the logic in all this and perhaps can explain it better :)