Mail keeps downloading thousands of old emails

I'm running an iMac with OS 10.10.5, (because I have vital software that might not survive an OS upgrade), and a week or so ago I noticed an activity message at the bottom of Mail saying 99 thousand or so emails were being added. In the meantime that old Yahoo account, that is still my primary email account, stopped adding any new emails. Now I have to check my email on my iphone in Mail. Or, I can just get used to going to Yahoo.com and stop using Mail. I've read through the responses to other issues resembling mine and I'm unwilling to delete the account and add it back because it seems like any time I'm instructed to do something like that from Apple employees lately, something unexpected and irreparable happens. I was in the process of cleaning up and organizing my mail not long ago and when I was deleting lots of old emails, and adding them to mailboxes, the whole program semi-crashed, flashed to nothing then came back, except my "receipts and confirmation" mailbox was gone, along with all the documents in it. It was only the most important mailbox I kept. Oh well. Now this. If anyone figured out how to stop the flood of email coming in, I'd be super thankful. Otherwise, if I keep using mail, I'll be spending lots of time deleting mail from year's past.