Mailbox.app crashes on startup every time. This happend after a Gmail account was disabled (account associated with previous job) before it was removed from Mailbox.app. This caused crashes on the iOS and OS X versions of the app that made them both unusable.
An updated app was relased at some point that allowed me to "delete" the prevoiusly deactivated account and has been working for a number of weeks.
It doesn't appear there has been a new version of the app in quite a while, but the crash reporter dialog finally clued me in to what to look for (or more accurately I paid attention to what they had said all along). In the report there is a string "com.dropbox.mbd.external-beta" that indicated what I should be looking for in the way of local preferences, settings, and cache files. (Previously I had poked around looking for "mailbox" or similar but didn't find anything)
$ find ~ -iname com.dropbox.*
/Users/cbeck/Library/Caches/com.crashlytics.data/com.dropbox.mbd.external-beta
/Users/cbeck/Library/Caches/com.dropbox.mbd.external-beta
/Users/cbeck/Library/Preferences/com.dropbox.mbd.external-beta.plist
/Users/cbeck/Library/Saved Application State/com.dropbox.mbd.external-beta.savedState
$ rm /Users/cbeck/Library/Caches/com.dropbox.mbd.external-beta
$ rm /Users/cbeck/Library/Preferences/com.dropbox.mbd.external-beta.plist
$ rm /Users/cbeck/Library/Saved Application State/com.dropbox.mbd.external-beta.savedState
$ rm /Users/cbeck/Library/Caches/com.crashlytics.data/com.dropbox.mbd.external-beta
Cool,thanks.You can only remove the cache data then everything will be ok.