Is there a process for rebuilding your Mail database

My son gave me a URL to help me clean up space taken up by files related to the Mail program (see: <https://macmyths.com/how-do-i-free-up-disk-space-on-my-mac/>). I tried to delete a large number of emails, but they went away and then came right back. After posting this question I was told that my problem had to do with something about the difference between POP and IMAP (?) servers and that I needed to clear out garbage on the ISP's email page; then that would clean the same things from my computer. That seems to work. Then I'm doing as VikingOSX suggests, below. Thanks

Way down at the bottom of the Mailbox menu. I missed it, Thanks for pointing it out.