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