Never saw the prior reply, but thanks for the information.
First, that's good you are making backups. However, Western Digital drives are somewhat notorious for not playing too nicely with Macs. Also, while Time Machine is a good backup program, it is not bootable, and thus recovery would be a chore. I myself prefer (and use) SuperDuper!. It makes a bootable backup/clone of my system (I use it on both of my Macs). It definitely makes both recovery and the clean installation of a Mac OS smoother).
Secondly, Malearebytes is excellent for detecting adware/malware. You should not really need Avast, as long as you are careful as to which websites you visit, and what software you download (and from where). Neither of those programs, though, are effective regarding disk cleanup/maintenance/repairs. Here are some excellent free programs that work well, will not do any harm to your system (as long as you use them correctly), and are reliable:
1.
Onyx - Available from here:
https://www.titanium-software.fr/en/onyx.html
A stellar product that is rock solid, has been around for a number of years, and has had numerous positive reviews. I use it religiously (along with the excellent commercial program Tech Tool Pro). In fact, it is one of my 6 critical programs that must be compatible with a new Mac OS before I move to the new OS. Make sure you get the correct version for the OS you are using.
2.
GrandPerspective: Available from here:
http://grandperspectiv.sourceforge.net/
Neat, graphical way of identifying large files.
3.
AppCleaner: Available from here:
https://freemacsoft.net/appcleaner/
Very good program for removing most (maybe all?) "stuff" associated with an application that you are deleting.
Also, look in your downloads folder. There could be stuff there you don't need.
Now, as for your original problem, do you happen to have the file entitled "Install MacOS El Capitan"? If you do, maybe a clean installation of OS 10.11.6 (along with "migrating"/copying all your needed "stuff" from your backup) will resolve the issue. I use Outlook 2016 for my EMail client, and I have never had an issue with it.
One other possible concern, though, is that if the machine still has the original internal drive, it is 9 years old. Without disk cleanup/maintenance/repairs, that could make matters worse. Not saying that is the case, but taking care of a Mac is just like taking care of an automobile. Neglecting to take care of an automobile with routine, scheduled maintenance (along with doing some on your own) will only lead to more issues. And that is just as true with automobiles manufactured by Honda and Toyota, the two most reliable brands (just like Macs).