The email notifications - max. volume

I’m having hard times with team members regarding their miss of the discussion notification.
Even if those people where mentioned in the discussion, and their session status is offline, it seems that notifications are not reaching some of the people involved in discussions.

Are there any estimations that can be made regarding the volumes that the app can handle?
I’v checked with the email delivery sistem and everything works fine as it is a smtp service powerful enough.
Thank You!

System Console > Log Settings > File Log Level > DEBUG will let you log email and push notification send events, and you can see if it’s an issue with the system or the email service?