Can anyone tell me if it is possible to set up notifications only for messages from a specific user or group?
This should be done by AquaMail plugin feature in Tasker (as StR said before). (But I did not test it)
You can specify, which messages (e.g. "from xxx") will cause actions.
And you will have many actions, (again by AquaMail plugin in Tasker) like enabling notifications.
Or you could just set Tasker to play a specific sound
@Paris Geek: I didn't see, that ReubenDiaz already knows other apps who do this, maybe he just wanted to ask a normal question?
And with AquaMail he found one of the mail apps, which provide a tasker plugin feature. Another app is K-9, btw.