Thanks for your response. I tried Safe Mode and also checked my paths on Mailbox behaviors. But it just happened again. This time, I was merely writing an email when it crashed. Here's part of the crash report. Any thoughts?
Application Specific Information:
*** Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[__NSArrayM objectAtIndexedSubscript:]: index 9223372036854775807 beyond bounds [0 .. 2]'
terminating with uncaught exception of type NSException
abort() called
Application Specific Backtrace 1:
0 CoreFoundation 0x00007fff31767a4b __exceptionPreprocess + 171
1 libobjc.A.dylib 0x00007fff589f4c76 objc_exception_throw + 48
2 CoreFoundation 0x00007fff317a8cd4 _CFThrowFormattedException + 202
3 CoreFoundation 0x00007fff31788ce0 -[__NSArrayM removeObjectsInRange:] + 0
4 IMAP 0x00007fff4b31183a -[IMAPMailboxSyncState messageNumberForUID:orRange:] + 175
5 IMAP 0x00007fff4b320972 __93-[IMAPNetworkTaskHandler connection:receivedUnhandledUntaggedResponse:forMailbox:forCommand:]_block_invoke + 45
6 Foundation 0x00007fff338226fc __NSIndexSetEnumerate + 633
7 IMAP 0x00007fff4b320850 -[IMAPNetworkTaskHandler connection:receivedUnhandledUntaggedResponse:forMailbox:forCommand:] + 1189
8 IMAP 0x00007fff4b30173e -[IMAPConnection _recordUntaggedResponse:forCommand:exists:receivedExists:fromIDLE:] + 4212
9 IMAP 0x00007fff4b2fbca3 -[IMAPConnection _copyNextTaggedOrContinuationResponseForCommand:exists:receivedExists:] + 1038
10 IMAP 0x00007fff4b30031a -[IMAPConnection _responseFromSendingCommand:receivedExists:] + 1386
11 IMAP 0x00007fff4b2fb5f1 -[IMAPConnection executeClientCommand:] + 60
12 IMAP 0x00007fff4b2fb45b -[IMAPConnection prepareAndExecuteCommand:] + 220
13 IMAP 0x00007fff4b32da94 -[IMAPServerInterface executeClientCommand:] + 1195
14 IMAP 0x00007fff4b32ba38 -[IMAPServerInterface expunge] + 56
15 IMAP 0x00007fff4b33b235 __45-[IMAPTaskManager expungeMailbox:ifSelected:]_block_invoke + 44
16 IMAP 0x00007fff4b33874b __89-[IMAPTaskManager _doCommandBlockForMailbox:block:async:ifSelected:priority:description:]_block_invoke_2 + 128
0x00007fff337f584c __NSOQSchedule_f + 342
25 libdispatch.dylib 0x00007fff595d4db8 _dispatch_client_callout + 8
26 libdispatch.dylib 0x00007fff595e7e81 _dispatch_continuation_pop + 472
27 libdispatch.dylib 0x00007fff595df733 _dispatch_async_redirect_invoke + 703
28 libdispatch.dylib 0x00007fff595d69a9 _dispatch_root_queue_drain + 515
29 libdispatch.dylib 0x00007fff595d6755 _dispatch_worker_thread3 + 101
Thread 0:: Dispatch queue: com.apple.main-thread
0 libsystem_kernel.dylib 0x00007fff5975520a mach_msg_trap + 10
1 libsystem_kernel.dylib 0x00007fff59754724 mach_msg + 60
2 com.apple.CoreFoundation 0x00007fff316e22c5 __CFRunLoopServiceMachPort + 341
3 com.apple.CoreFoundation 0x00007fff316e1617 __CFRunLoopRun + 1783
4 com.apple.CoreFoundation 0x00007fff316e0c93 CFRunLoopRunSpecific + 483
5 com.apple.HIToolbox 0x00007fff309cbd96 RunCurrentEventLoopInMode + 286
6 com.apple.HIToolbox 0x00007fff309cbb06 ReceiveNextEventCommon + 613
7 com.apple.HIToolbox 0x00007fff309cb884 _BlockUntilNextEventMatchingListInModeWithFilter + 64
8 com.apple.AppKit 0x00007fff2ec7ba73 _DPSNextEvent + 2085
9 com.apple.AppKit 0x00007fff2f411e34 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 3044
10 com.apple.AppKit 0x00007fff2ec70885 -[NSApplication run] + 764
11 com.apple.AppKit 0x00007fff2ec3fa72 NSApplicationMain + 804
12 libdyld.dylib 0x00007fff5960e015 start + 1