Why does email open with deeplink URL?

Question

I'm posting a question here because I don't know where else to turn. Why does Outlook open some emails in a "deeplink" window instead of just showing the full-page email like it's supposed to? I've tried to find help on other sites, but get no answers or I'm told I'm on the wrong forum and the question is removed. I'm using Google Chrome and have no mobile devices connected to this acct. I've sent dozens of emails to Outlook Support and Microsoft which were useless. They either don't know what to do or won't say, as though deeplink was some big secret.

The email URL usually starts with https://outlook.live.com/mail/0/deeplink?version . . . . etc. and opens in a new, smaller window. Some emails do it and some don't, even when they're from the same sender. I just want to know what's causing this and how to stop it or turn it off. I want emails to open on the full screen where they're easy to read. Is there some program or software that I'm missing; something I should remove? Change settings? Has someone hacked into my acct.? I'm lost and about ready to move to a new email provider (any suggestions?).

I tried all kinds of "fixes" as instructed by Outlook Support and none solved the problem. It was a waste of time, so I gave up. I don't understand computer code and don't expect a lengthy explanation, but if anybody can just tell me what to do or where to look, I'd really appreciate it.