This is odd because I also use Outlook 2016 and just to be sure, I had a Pizza Hut promotion email in my Outlook inbox. I clicked on a link and Edge (my default browser) opened up just as expected - not IE.
I would go to Start > Settings > Apps > Default apps and make sure your preferred browser is properly set.
You could even go through the long list of file types under "Choose default apps by file type" to make sure IE does not show up where unwanted. Now it will show Internet Explorer for .mht, .mhtml, .partial, .website, .xrm-ms, and .xsl files as those are IE exclusive file formats. But they are (or should be) rarely if ever used. So it should not matter they are set to IE here- unless your email links use those file formats. And if they do, then that's on the sender for being lazy and using those obscure file formats, not your system.
I note if you have already changed your default browser to something other than IE and you still find individual formats set to IE, that is just an indication as to how rare and obscure those file formats are - or else your new browser would have changed them when it was set to the new default.
You may also find a bunch of Microsoft Visio Document file formats that open with Internet Explorer or a dedicated viewer you get from the MS Store. That's okay as Visio is used (or more accurately, "was" used) to make drawings and such inside Internet Explorer documents. This was fine when IE dominated the Internet, but no longer. So leaving them to IE is fine.
In addition to those above, I found .svg file format set to IE but a little research shows Edge will open it, so I manually changed it to Edge, but I doubt I will encounter any files using that extension.
One last thing to try is to go into the settings page in your preferred browser and select the option to make it the default. Don't be surprised if it sends you back to Start > Settings > Apps > Default apps.
