Mail Will Not Rebuild

When opening the mail app a box pops up telling me I need to quit and rebuild. When I quit and open mail again (it says to open mail again and mailboxes will automatically rebuild) everything freezes again and again a box pops up telling me to quit, open mail again and it will automatically rebuild. This becomes an endless loop. Mail is currently unusable. I am running OS X El Capitan version 10.11.6 and am up to date on any mail updates. How do I get out of this endless loop and get Mail working again? (also, I tried to delete and reinstall but you cannot delete Mail) Thank you