Mail.app opens spontaneously
I found that this worked for some users:


  Open and login to Google Calendar (https://calendar.google.com)
  
  Select the Setting gear icon (upper right-hand corner), then select
  “Setting” from the pull-down menu
  
  Below the “Calendar Settings” header select the Calendars link
  
  To the right of the calendar name under NOTIFICATIONS, select Edit
  notifications
  
  There are five options in the section “Choose how you would like to be
  kept up-to-date:” - remove all of the checkmarks under the Email
  column
  
  Click the “Save” button at the bottom of the page