email client issue with office 365

As a preface, let me say that I was surprised you would replace an email system that supported standard protocols (IMAP and POP) used by dozens of clients and which worked well with all of them with a system that required a proprietary protocol and made you captive to a single client. I tried Outlook for some time. Besides serious shortcomings in functionality and ease of use compared to popular clients like Thunderbird and mail.app, it has security issues that make me wonder why you even let people use it. (One is that there seems to be no simple way to get it to always display the senders address: it only shows a name. One phishing scheme frequently observed on-campus is the email from your department chair, asking you do do something. If the sender's address is in Kazakhstan, that's an immediate reminder that this probably isn't really your department chair. But in Outlook, you don't see this.)

So I have gone to using mail.app (particularly since Microsoft claims that Office 365 supports IMAP). This works perfectly for months at a time (initially for more than a year). But there are occasional 'outages', lasting from a day to a couple of weeks, where there is some sort of loss of return communication with the server. Symtpoms: outgoing mail works fine. Incoming mail shows up normally and promptly in the Inbox. But if you move mail from the inbox to another folder (most of my folders are on the server but have local copies for reference when I'm not online) or delete them (which I guess is actually the same thing since it just moves them to a "trash" folder), an hour or so later they will all show up back in the inbox. Needless to say this is pretty frustrating when you've just spent a couple of hours sorting them out. My local copies of some folders also get messed up sometimes, with messages missing entirely, blank, or missing attachments. Most of this gets fixed once mail starts working normally again.