some of my users use Outlook (of course) and the can manage multiple accounts /email adresses with all subfolders, calendars etc.

With SM webmail that is not possible in the same way. Of course one can share a single folder, a calendar, etc but what if you want to share all resources in your account with someone else.
I think it's an advantage when a user can share his account with an other user (within the same domain) and that the target user can switch between acccounts.

It wouldn't be like outlook where you have all the accounts in one screen, but to be able to switch between accounts would be nice.

maybe with exclusions not to be able to change settings like password or to reshare that account with someone else.