Cannot Verify Server Identity

I had not used the Apple Mail app for a long time, instead using the Gmail app. I was recently required to use the Apple Mail app as I needed to add a school domain that was not supported by Gmail. At some point along the way I accidentally told the Mail app I was adding an Outlook account, even though I was not. And ever since, I receive about one to four "Cannot Verify Server Identity - The identity of "Outlook.office365.com" cannot be verified." pop-ups upon each pick-up. The only options are "cancel" and "details." There is nothing useful in "Details," or at least not anything I can understand. This is really starting to bother me and I'm not sure how to fix it.