You can't "call up" from a trigger to a XS engine functionality.
And there's no direct way to send emails out from trigger functionality.
In fact, that's not your requirement. You requirement is to send an email-notification when some sort of business event occurs. This business event is represented by the data in the table - but it's not the event itself.
So, your solution should not handle the warnings on a higher level.
For example you could write a stored procedure that enters the data into the table and that has a return parameter to indicate that the new "Stage" level is "Critical".
Your application could then trigger whatever action is required.
- Lars