I've been reading about your problems and tried to follow what could help me too, but it's basically impossible.
Mail was the one app that couldn't sync properly after installing El Capitan, and the procedure failed twice before finally succeeding. when it did I saw dozens upon dozens of emails coming through from all my accounts (I have an iCloud and a couple of gmail accounts), many already following rules and going in their separate folders so I thought "no big deal, I'll just have to mark them as read and be done with it".
the problem is the app lasts mere seconds before crashing, I almost simultaneously get a notification to enter the password for one of the gmail accounts: I do so, I get "couldn't verify password or account" every time.
I deactivated the account to see if that's what making Mail collapse so suddenly, no happy ending. I literally have mere seconds, so I cannot uncheck "Automatically detect and maintain account settings", it's simply impossible.
should I wait for them to fix the issue or is there something in those 11 seconds or from outside the app I can try? thank you!
this is the post-crash message I get:
Process: Mail [568]
Path: /Applications/Mail.app/Contents/MacOS/Mail
Identifier: com.apple.mail
Version: 9.0 (3094)
Build Info: Mail-3094000000000000~1
Code Type: X86-64 (Native)
Parent Process: ??? [1]
Responsible: Mail [568]
User ID: 502
Date/Time: 2015-10-03 08:16:00.605 +0200
OS Version: Mac OS X 10.11 (15A284)
Report Version: 11
Anonymous UUID: 365E01B2-EF2C-D3A0-851E-9828A5000DA5
Time Awake Since Boot: 1100 seconds
System Integrity Protection: enabled
Crashed Thread: 8 -[AttachmentManager _synchronouslyPurgeAttachments] Dispatch queue: NSOperationQueue 0x7fc253ca1bd0 :: NSOperation 0x7fc25908a330 (QOS: UTILITY)
Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY
Application Specific Information:
*** Terminating app due to exception while holding database lock 'NSInternalInconsistencyException', reason: 'Cannot set MFGmailLabelStore as data source for message.'
abort() called