Mail crashes in 10.11 Beta (15A244d)

Could you post a short error log?

I had initially the same problem. It turned out that after reconnecting with my (iCloud based) keychain and disable and reabable my email accounts everything is back to normal.

Can you describe that process? I turned off and on the keychain in the iCloud preference pane, and disabled and re-enabled the accounts in "Internet Accounts" to no avail.

Mail has serious problems with EXCHANGE accounts - since BETA 1 nothing get better.
I installed everything new, same problems with EXCHANGE.

So MAIL 10.11 Beta 6 is not working correctly.

You can diagnose if this being caused by an issue with one of Mail's resource files by forcing Mail to rebuild them (this has nothing to do with rebuilding a mailbox). Quit Mail and move the following folder to the desktop (you can press shift+cmd+G from Finder and copy-paste if you need to):

~/Library/Containers/com.apple.mail

Open Mail again and, if it needs to, let it re-migrate. Hopefully the issue won't reoccur.

If it works, then you can delete the folder from the desktop, but you'll need to re-do any special settings you may have had applied in Mail before.

If it doesn't work then you can replace the new com.apple.mail folder with the one on the desktop to preserve your old settings.