Upgrading from 10.10 Yosemite to 10.11 El Capitan is as easy as usual but there is one glitch that haunted me for a few days. Mail.app crashes on launch. Turns out wrong settings with IMAP is the cause.
Unlike Yosemite is more tolerant with “wrong” settings in Mail than El Capitan.
I need to first disable all network. Disable all IMAP accounts.
Go through all my IMAP accounts and enable one account at a time and turn on network.
If Mail crash, there’s something wrong with the settings.
The fix is removing the value in IMAP Path Prefix from some of my non Gmail accounts.