First, have the AC users validated their accounts by replying to the validate emails? If not then they need to validate their accounts.
Second, restart AC.
2. login with admin:admin
3. click the SolarWinds Mgmt tab
4. click the "Restart Alert Central" button
Thanks for the reply! The users are validated I tried restarting the server but the same thing continues to happen. Seems to work fine from other email sources just not this one.