It's an imap account. You may have seen my subsequent post saying it stopped doing it and is acting normal now. I am concerned that when I get back to my newer Mac after d/ling Catalina, my email there will be doing the same thing only not quite so many emails because it's only been a day or two since I've used that account not that Mac. On the older Mac, I quit Mail a long time ago because I don't use it there, and then it caught up with me for some reason, even though the server doesn't keep them after I read them on my Mac mail program. Who knows?
Is your email account set up as IMAP or POP? (To check open Mail > Preferences > Accounts > Server Settings and see if the INCOMING Mail Server is IMAP or POP.)
