Pop mail won't delete from server after 1 week per Sierra Mail account settings

I've been having trouble for awhile with my sbcglobal.net email account on my iMac 27" Retina 2015 running Sierra OS.

My mail settings on the account for "Remove copy from server after retrieving a message" are set at "after 1 week", because I do also access this email account on my laptop and IOS devices. This strategy of managing my Mail has worked consistently for me for years across Mac OS versions but has become problematic for me the last several months.

Today, despite downloading mail on my iMac today and typically at least once a week, I have 5,363 messages on the server dating back to 6/23. Trying the "Remove from server" in the account window does nothing, even if I try to remove messages from the list only a week or two's worth at a time.

Has anyone had this problem? I am wondering if upgrading to Mojave or to High Sierra (which I have available - I've got the installer on a flash drive so I have my choice of both OS to go to) would help.

I may also look into whether I can configure this account as IMAP which I think I'd like better.

Thanks in advance.