So first of all, I did my first mistake by buying a domain on Onlydomains.com. 

Then I created the same domain for an e-mail through Microsoft Online (Office 365?) and a university license.

I proved that the domain is mine, I also added MX, A-Track records, CNAME and connected gmail POP3 to this account I created.

It worked fine for some 2 months or so but then I suddenly stopped receiving mail, it said something about POP3 configuration error.

I did what I could but it led me nowhere. I guess I configured the POP again, all mail disappeared in Outlook and now rarely some received mail appears. That includes verification mail from Gmail so I can connect them again. An error shows up that says - https://support.google.com/mail/answer/7720

Now I'm stuck and I don't know where it's gone wrong.
A-Track, CNAME, MX? 

Office 365 Can't receive mail