I've tried to sign up for a mailing list this week and never got a confirmation. Also, when requesting my password from another site I'm not getting that either. Both using existing addresses with high numbers of emails remaining. Neither has any deleted messages either.
The number of emails I've gotten from SG addresses has also seen a big drop this week. I've gotten 1 or 2 from lists where I normally get around 10. I had this same problem not very long ago. I can provide more details if needed.