Tons of mail windows opened because of no preferred outgoing server
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.

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