Ask HN: My employer hacked/accessed by email and recalled email. What do I do?

I work in an academic establishment in the UK and recently, I have started to have some issues with my manager. To cut the story short, In an exchange of emails, I had pointed out how incompetent he has been and I had copied 3 of my team members in this email, (while I was away from my work place - accessed from my iPhone.) Workplace uses office 365 email service. Now, my manager or 'someone' at work, has got into my email box and 'recalled' this email I had sent. Obviously, it didn't work. As a result, all my access has been blocked (I presume my password got reset? I am not sure) and now, I am not able to get into my emails or the office 365 service. Currently, I am away from work, and will return on Thursday. I have a few questions to this end: (1) how can I conclusively prove that my manager or someone at work has gone into my email and recalled my email? (2) how did I approach this problem?

It is pretty depressing for me that an educational establishment has done such a nasty thing.