If the Office365 account was provided by your employer, it's theirs, not yours, and you have no particular rights over email communications conducted as part of your employment. Why would you expect to continue to be able to use an account they are paying for after you've stopped working for them? If you were using their account for your personal communications, that was a poor choice by you, and probably prohibited by your employment contract anyway. Their GDPR basis for processing will be contract or statutory requirement (not consent), and there will be a ton of case law on this kind of thing.
If you were using a personal account for business, they will have no rights to that, no ability to reset your password, nor access your email, however, you may have broken your contract if you used it for work-related email, for which they may have recourse.
There is no abuse of privacy here. If you want further advice, ask a lawyer, not Stack Overflow.