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for some days we had massive problems with Apple Users who migrated to El Captain, using Apple Mail.
IMAP sync was was running all the time, blocking the whole system.
At the server we could see massive filetransfer (50-70 Mbit per user) in the background.
Update 14.4.5784 fixed the issue that the users can´t work anymore, but still massive data transfer is running in the background. Though the problems got better after the update, we think that it concerns the RFC change in the fix.
Somehow there might be a reference with: the Topic "upgrade to 14.4.5784 IMAP folders not re-syncing"
Other users/accounts run pretty well - only El Captain Users have this issue so far.
Please excuse my bad english - it is not my native one.
Help or any ideas on this are appreciated.