Impersonation allows you to access another mailbox if you must, otherwise rely upon the user playing ball and leave it shared with you.

That doesn't cut it in some environments though - let's say a school environment where safeguarding is really important. Having measures like opening another mailbox would be useful, and students will not leave it shared - that's just a fact!

In the case you sight you should probably be thinking Microsoft 365 / Exchange Online with the level of control and functionality it provides. Particularly in terms of the type of access control that you need in the specific scenario you describe and the likely audit / classification / compliance / journaling requirements in an educational setting to achieve a robust safeguarding capability. M365 will almost certainly be on the approved product list with any UK schools / educational trusts / authorities too, which I suspect will not be the case with Smartermail.

Smartermail is a good product and it can do some of what you can achieve with Exchange on-premise or Exchange Online but in terms of depth and breadth of functionality they are two different propositions. Clearly Smartermail is infinitely cheaper and it does a lot of what Exchange can do so does have its use cases, which is why we use it for some scenarios.

Completely appreciate that POV. We currently use 365 for all of our clients. We was sold a package today to trial (SmarterMail), and was told 'it's now an alternative to Exchange'. Therefore, I've gone in looking for these features. I'm not too heartbroken that they're not there, but I thought the only way that the product will improve is if we cast these views.

It's not something I expect to see in the software, it's just something that would be nice to see in future, if considered (as with all my suggestions). 

We will use SmarterMail for some use cases - it seems very robust and reliable. :)