Despite the age of the protocols, you can cheerfully connect a wide range of IMAP4 and POP3 clients to Exchange Online. If you do, you might need to consider how to handle calendar appointments, and if you want to use iCAL, you’ll need to make some adjustments with PowerShell.
Exchange Online supports inactive mailboxes as a way to keep mailbox data online after Office 365 accounts are removed. Inactive mailboxes are available as long as a hold exists on them. You can update mailbox properties to exclude all or some org-wide holds. If you exclude holds from a mailbox, you run the risk that Exchange will permanently remove the mailbox. If that’s what you want, all is well, but if it’s not, then you might not be so happy.
Teams allows users to send email to channels via special email addresses. Those addresses aren’t very user-friendly, but you can add them as mail contacts so that channel addresses show up in the Exchange GAL. It’s easy to do and makes it much easier for people to email Teams channels. That is, until someone removes the channel email address…
Outlook Mobile clients can now schedule Teams meetings, even if your tenant isn’t using the newer version of Outlook’s mobile connection architecture. The Office 365 tenant setting for Skype for Business Online co-existence mode has to be configured to use Teams, and once everything is in place Outlook is happy to schedule Teams meetings.
You can use Exchange Address Book Policies (ABPs) to limit the ability of Teams users to chat with each other. Everything works as expected until you look for some new teams to join only to find that Teams can’t suggest any teams to you. The problem seems to be with filtering the set of teams returned by the Microsoft Graph to take account of the scope applied to the user. At least, that’s what I think is going on.