I use El Capitan Mail and have a dozen or so imap accounts in addition to a few pop accounts. I only converted the imap accounts from pop a few days. Since then, Mail has been hanging, crashing, not filing or deleting messages without them returning to the inbox on restart, not syncing properly with my devices, and using CPU resources that are almost always at least 200% and many times as high as 450% (4 gHz i7 5K iMac, OSX 11.2), causing overheating and fans to run extensively.
This has also resulted in 100GB of Mail log files being generated every 24 hours!
All these issues disappeared when disabling Avast Mail Shield.
I would be happy to provide more information if desired. Meanwhile, I'd sure appreciate a fix for Avast Mail Shield so that I may enable it again.
I have examined Console and found hundreds of Avast errors. This is a sample. In just one minute there were 135 of these!
1/25/16 2:15:51.000 PM kernel[0]: com.avast.fileshield Warning: Unable to send req. /Users/todd/Library/Containers/com.apple.mail/Data/Library/Logs/Mail/2016-01-25_IMAPConnection-santro.websitewelcome.com.log [40]
1/25/16 2:15:51.000 PM kernel[0]: com.avast.fileshield Warning: Unable to send req. /Users/todd/Library/Containers/com.apple.mail/Data/Library/Logs/Mail/2016-01-25_IMAPConnection-santro.websitewelcome.com.log [40]
1/25/16 2:15:52.000 PM kernel[0]: com.avast.fileshield Warning: Unable to send req. /Users/todd/Library/Containers/com.apple.mail/Data/Library/Logs/Mail/2016-01-25_SocketStreamEvents.log [40]
1/25/16 2:15:52.000 PM kernel[0]: com.avast.fileshield Warning: Unable to send req. /Users/todd/Library/Containers/com.apple.mail/Data/Library/Logs/Mail/2016-01-25_IMAPConnection-santro.websitewelcome.com.log [40]
1/25/16 2:15:53.000 PM kernel[0]: com.avast.fileshield Warning: Unable to send req. /Users/todd/Library/Containers/com.apple.mail/Data/Library/Logs/Mail/2016-01-25_Library.log [40]
1/25/16 2:15:53.000 PM kernel[0]: com.avast.fileshield Warning: Unable to send req. /Users/todd/Library/Containers/com.apple.mail/Data/Library/Logs/Mail/2016-01-25_SocketStreamEvents.log [40]
1/25/16 2:15:54.000 PM kernel[0]: com.avast.fileshield Warning: Unable to send req. /Users/todd/Library/Containers/com.apple.mail/Data/Library/Logs/Mail/2016-01-25_Library.log [40]
1/25/16 2:15:54.000 PM kernel[0]: com.avast.fileshield Warning: Unable to send req. /Users/todd/Library/Containers/com.apple.mail/Data/Library/Logs/Mail/2016-01-25_Library.log [40]
1/25/16 2:15:55.000 PM kernel[0]: com.avast.fileshield Warning: Unable to send req. /Users/todd/Library/Containers/com.apple.mail/Data/Library/Logs/Mail/2016-01-25_SocketStreamEvents.log [40]
1/25/16 2:15:55.000 PM kernel[0]: com.avast.fileshield Warning: Unable to send req. /Users/todd/Library/Containers/com.apple.mail/Data/Library/Logs/Mail/2016-01-25_SocketStreamEvents.log [40]
1/25/16 2:15:56.000 PM kernel[0]: com.avast.fileshield Warning: Unable to send req. /Users/todd/Library/Containers/com.apple.mail/Data/Library/Logs/Mail/2016-01-25_SocketStreamEvents.log [40]
1/25/16 2:15:56.000 PM kernel[0]: com.avast.fileshield Warning: Unable to send req. /Users/todd/Library/Containers/com.apple.mail/Data/Library/Logs/Mail/2016-01-25_IMAPConnection-santro.websitewelcome.com.log [40]