Very strange!!
Usually it will show in the server errors section if an email had trouble being sent, but I do not see one for you recently, so this is really strange.
We will get this figured out!!
EDIT:
It did in fact show in server error log that email to you failed yesterday at 6:48 AM. No valid recipients.
EDIT 2:
It is showing in the email bounce log, today at 2:25 AM "Too many soft bounces"