i've been looking answers for the same problem, i get "Messages stuck in the queue" and while i was trying i notice something that could be the solution (centainly not the professional answer):
1. click start, control panel, administrative tools, IIS services
2. expand PC and right click "default virtual SMTP server"
3. select delivery tab (spanish: ENTREGA)
4. set lower values for: "first retry interval", "second try interval", "third retry interval", "subsequent retry interval"... (i set them to 1)
now it's working fine for and hope this to be usefull