Mac Mail in El Capitan

I've been having all sorts of problems lately with Mail in El Capitan.

First, Mail started crashing every time I tried to access it. Following a suggestion I removed Com.Apple.Mail folder and com.Apple.MailServiceAgent folder from “~/Library/Containers”

to the desktop and restarted. The first time I did this it solved the problem for a couple of days, but then the crashing started up again. I did it a second time and, so far, it seems to have worked.

Second. I am now getting an alert and I can't receive or send mail in the Mail app.

I can send and receive mail via iCloud.

It's my understanding that I cannot just reinstall Mail and that I must reinstall El Capitan. However, for some reason, when I start up in macOS Recovery mode I'm unable to reinstall.

I'm at a loss as to what to do. Can anybody give me a clue?