Tons of mail windows opened because of no preferred outgoing server

Solution 1:

1. Trying to send you emails because, email is one of the options for Reminders in Calendar.

   • To get rid of this for existing events, simplest is just to manually one-by-one change the reminder setting. Alternative would be (1) export the calendar to a file, (2) Make a backup copy for when you mess up step 3 and regret it bitterly at step 4, (3) edit the file to search and replace reminders (4) delete your calendar (5) import the edited events.
   • To get rid of this for future events, I think you just have to change the reminder setting once; on your one next new event. Then it will remember.

2. Force-quit with ⌥⌘esc. If that really really doesn't work, logout.

3. Worst case, avoid the future annoyance either by telling the Mail app your email details; or open Mail app -> Preferences to tell it what program you actually do use for email.