Mail on El Capitan totally messed up. Random failure every few minutes with detailed report to Apple in window with restart option on selecting "Report". Opens again, but when messages sent, they show up in Outbox instead of Sent file. Contacting comcast.net, my server, notes they went out, but Mail isn't showing them as sent, only in Outbox. Sending mail from my iCloud address from my iPad Air 2 to my comcast address works fine, but not vice versa. Messages sent from my Mail app from my iMac get to the iCloud address and to my iPad, but Mail on the iMac just show the message in Outbox and not in Sent file. The obvious culprit is Mail, since everything else works, and the multiple report to Apple as usual disappear into the universe somewhere. Problem apparently due to large number of messages held in mailboxes and lots of rules. Cleared several hundred messages held in Sent and Inbox. Trashed all rules filtering lots of SPAM. Cleared down to about 40-50 in each of the boxes and all rules and Mail erratic behavior and spontaneous quitting have stopped. Question: Why? Unfortunately, SPAM also returned. Apple thinks SPAM is acceptable and limiting anti-SPAM rules?