SendFromCron is periodically called from cron, figures out which it needs to send, sends them, and records they've been sent in the datastore.

SendKeyExpiredDeleted sends an email to the given address informing them that their key expired and was deleted.

SendKeyExpiresEmails sends expiry reminders for keys expiring in 14, 7, 3 days.

SendTestEmails sends a plaintext and an HTML email to the given to address.

SendVerificationEmails iterates through the email addresses on the given key and works out whether to send each one a verification email.