If it is an advanced alert you can open the alert in "Alert Manager" ("Start->All Programs->SolarWinds Orion->Alerting, Reporting, and Mapping->Advanced Alert Manager") and the last tab when editing an alert will be the alert "Reset Action", here you can configure the reset action or edit the current reset actions.
If it is another type of alert it can be editted in the same manner, in the "Alert Reset" tab.
A few things to check for is that any email "Alert Reset" actions exist and if they do if they have valid SMTP details and the correct address to send to. Copying most of the setting from the working "Trigger Action" is probably the best way to get it working as you need.
All the best,
The SMTP address was different and we had recently changed our SMTP ip address, hence we were not able to receive resolved alerts.