Use active tab as Default Sender
Added option 'Use active tab' to Default Sender preference (Preferences > General). It does always use the account of the currently selected tab to compose messages (e.g. via mailto:).
Block Google Calendar Alerts
'Block Google Calendar Alerts' (Preferences... > Accounts > Advanced) suppresses notifications coming from Google Calendar. It helps to prevent getting notified multiple times for the same event (e.g. if the Apple's Calendar app is already notifying).