Mail crashes after it starts up

Out of the blue (no recent computer updates or changes) Mail crashes either immediately after I start it up, or when I click on anything in the Mail window. Have tried restarting computer with no success. Running OS 10.11.1 and can't update it till I get on a faster connection. Below is part of the report I get when it crashes. Any suggestions?
Thanks,
Thom

"Crashed Thread: 7 -[MFMessageRouter _routeQueuedMessages] Dispatch queue: NSOperationQueue 0x7fb2195d50a0 :: NSOperation 0x7fb2194efbd0 (QOS: UTILITY)

Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY

Application Specific Information:
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSArrayM insertObject:atIndex:]: object cannot be nil'
abort() called
terminating with uncaught exception of type NSException"

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