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Give the app plenty of time to download all messages until you no longer see "Checking for Mail." at the bottom left of the app screen (may take a few minutes depending on how many messages you have on the server).
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Before opening the Mail app, turn off your wifi connection (prevents mail from trying to download mailboxes from all accounts at once).The initial download of all messages from all accounts seemed to be overloading the app, causing it to crash.
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It's seems to have been related to the fact that I have multiple email accounts active in the app, as many people do. I had the same problem, but I believe I've fixed it. Should you be in the same position as the author of this, and have multiple mail accounts active, try the following steps: There is a good work around for this, that can be found on the apple discussion forum, here: Terminating with uncaught exception of type NSException *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Attempting to perform a synchronous operation while on the serialization queue' Here I am copying the beginning of text from the crash report-Crash is at Thread 22 (it is not whole text because of restriction of limited number of characters(if needed I will send it via email)): Process: Mail Would be nice if someone could help me with that.Thank you! I started to have this problem right after I installed El Capitan to clean HD and migrated within Migration assistant my old account and settings (everything except of applications). Every time just a one or two right after opening. Apple Mail app crashes for me nonstop after I updated to El Capitan.