Apple Mail automatically deletes then re-downloads thousands of emails each day

I'm using an Apple MacBook with macOS Big Sur 11.6.8 and Apple Mail version 14.0

For about 2 years I've been using Mail on my Apple MacBook for emails. Currently I have rather a lot of emails in my Inbox (25,200 messages). Each morning I open Mail and immediately about 18,000 of those messages disappear from Mail. It seems that Mail automatically deletes them. Then throughout the day Mail says it's downloading 18,000 messages. Eventually by the end of the day it has downloaded all the messages that go missing first thing in the morning. I'm not sure where it's downloading those emails from, as the emails it's downloading date back over a year and I'm sure are no longer on the webmail server of my internet provider. It takes about 10 hours for Mail to download the 18,000 messages each day which is inconvenient as I cannot refer back to emails that I need to for work if they are still downloading. It is also disconcerting to have thousands of emails spontaneously disappear without knowing why.

Is there any explanation why this is happening? I would prefer Mail to just download the 20 or 30 new emails that I receive each day and not 18,000 emails every single day, if that is possible?