I found a thread discussing this issue occurring in the 10.11.1 beta but I have not seen any threads or posts about this issue in the 10.11.1 official release. Upgraded to 10.11.1 yesterday. Mail.app crashes on startup. It only crashes if there is an email account enabled. My iCloud account is the only email account on this particular machine. If the email account is enabled, Mail.app will crash on startup. If I go into System Preferences and disable the email account, Mail.app will open, but there is no email account enabled so it's useless. If I disable the email account, open Mail.app, then go into Mail preferences and enable the account, it crashes immediately. If it matters this is a Early 2011, 13-inch, Macbook Pro, 2.7 GHz i7, 8 GB RAM, 10.11.1 is installed on a 250 GB internal SSD, and there is a 1 TB HDD in the optical drive bay. Thank you.