I don’t know why, but this is a standard issue. Even I had this problem. The Outlook never seemed like supporting the POP server of my account. I tried the same settings in the Mac Mail that worked effortlessly. An issue with Outlook.
You can switch from Outlook to Apple Mail for the same reason. I find it more soothing.
Outlook exports in OLM while the Apple Mail imports in MBOX. If you are archiving mails to the local storage, you will need to use a converter for importing the OLM files to Mac Mail format.
OLM Converter Pro is something, I will recommend. https://www.gladwev.com/portfolio/olm-converter-pro/