I would like to get an e-mail response report to check that our call-center employees actually meet our SLA: are all received emails answered in the correct time, or does it take too long?
The Message Response Times report template would meet your requirement. You will find it in the Templates > Mailbox Content menu.
It gives, mailbox by mailbox, the number/percentage of items without action, marked as reply, marked as forward, marked as unread with the average time to answer. It can drill down to the message level.