This was a fun one. Mail started crashing on launch for absolutely no apparent reason – nothing had changed to its config or similar in a long time. The crash logs were all fingering an identical culprit – -[IMAPTaskManager secondaryIdleMailboxName] called on the wrong GCD queue:
Process: Mail  Path: /Applications/Mail.app/Contents/MacOS/Mail Identifier: com.apple.mail Version: 11.3 (3445.6.18) Build Info: Mail-3445006018000000~4 Code Type: X86-64 (Native) Parent Process: ???  Responsible: Mail  User ID: … Date/Time: 2018-04-19 16:44:45.717 -0700 OS Version: Mac OS X 10.13.4 (17E199) Report Version: 12 Anonymous UUID: … Sleep/Wake UUID: … Time Awake Since Boot: 94000 seconds Time Since Wake: 530 seconds System Integrity Protection: enabled Crashed Thread: 13 Dispatch queue: Task Manager Serialization Queue (QOS: UNSPECIFIED) Exception Type: EXC_CRASH (SIGABRT) Exception Codes: 0x0000000000000000, 0x0000000000000000 Exception Note: EXC_CORPSE_NOTIFY Application Specific Information: *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'This method should only be called on the serialization queue' terminating with uncaught exception of type NSException abort() called Application Specific Backtrace 1: 0 CoreFoundation 0x00007fff55da96bb __exceptionPreprocess + 171 1 libobjc.A.dylib 0x00007fff7d4c1942 objc_exception_throw + 48 2 CoreFoundation 0x00007fff55daf2a2 +[NSException raise:format:arguments:] + 98 3 Foundation 0x00007fff57ee7340 -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 193 4 IMAP 0x00007fff6fe88959 -[IMAPTaskManager secondaryIdleMailboxName] + 216 5 IMAP 0x00007fff6fe880cb -[IMAPTask mailboxIsUserVisibleUsingDataSource:] + 180 6 IMAP 0x00007fff6fe6ab68 -[IMAPMailboxSyncTask _nextNetworkPriorityAndOperation:reservedNetworkPriority:] + 164 7 IMAP 0x00007fff6fe6c4ba -[IMAPMailboxSyncTask recalculatePriorities] + 398 8 IMAP 0x00007fff6fe67dd9 -[IMAPMailboxSyncTask initWithDataSource:taskManager:imapMailbox:fromStatus:forceFullSync:] + 766 9 IMAP 0x00007fff6fe931f7 -[IMAPTaskManager _syncMailboxWithDataSource:withIMAPMailbox:fromStatus:forceFullSync:userInitiated:] + 370 10 IMAP 0x00007fff6fe92e68 -[IMAPTaskManager syncMailboxWithDataSource:withIMAPMailbox:fromStatus:forceFullSync:userInitiated:] + 240 11 IMAP 0x00007fff6fe9631a -[IMAPTaskManager didAddMessagesWithUnknownUID:toDataSource:] + 872 12 Foundation 0x00007fff57e4a5df __NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 7 13 Foundation 0x00007fff57e4a441 -[NSBlockOperation main] + 68 14 Foundation 0x00007fff57e488ee -[__NSOperationInternal _start:] + 778 15 Foundation 0x00007fff57e44917 __NSOQSchedule_f + 369 16 libdispatch.dylib 0x00007fff7e09ee08 _dispatch_client_callout + 8 17 libdispatch.dylib 0x00007fff7e0b1ed1 _dispatch_continuation_pop + 472 18 libdispatch.dylib 0x00007fff7e0a9783 _dispatch_async_redirect_invoke + 703 19 libdispatch.dylib 0x00007fff7e0a09f9 _dispatch_root_queue_drain + 515 20 libdispatch.dylib 0x00007fff7e0a07a5 _dispatch_worker_thread3 + 101 21 libsystem_pthread.dylib 0x00007fff7e3f0169 _pthread_wqthread + 1387 22 libsystem_pthread.dylib 0x00007fff7e3efbe9 start_wqthread + 13
Long story short, the issue turns out to be having connection logging enabled. That’d been turned on many months before in order to debug a different stupid Mail bug, and had been simply left on (deliberately IIRC, since Mail tends to bug-out quite often, so why not have logs already available when it comes time to debug it yet again?).
Connection logging is enabled or disabled by opening the “Connection Doctor” (Window menu > Connection Doctor) and using the checkbox titled “Log Connection Activity”.
So how do you get to that checkbox when Mail crashes on launch? Well, in this specific instance I was able to disable all mail accounts via System Preference’s Accounts pane, launch Mail, disable logging, quit Mail, re-enable all mail accounts via System Preferences, and then relaunch Mail to have it finally actually work.
From even just brief web searching, it’s clear that this issue has been present and well-known in Mail for a really long time. Sigh. Apple’s protestations that they care about software quality, or the Mac, are relentlessly undermined by their actual actions.