Quite simply, I have Google Calendar set to send me e-mail notifications, and some of them are being outright bounced by Gmail, which then, ironically, has Google's Mailer-Daemon e-mail me about the bounced message, giving me a notification of sorts.<p>Any idea what the heck is going on with Google?
Do you have the bounce message?<p>It could have been your mailbox was unavailable (full? Overloaded by someone repeatedly refreshing the webmail page?). When a mailbox is unavailable, even briefly, you'll start to get mailer Daemon messages.