I never got this before (the following form) and had to look it up. I have a sign-up area and activation emails are sent. I have not reached the quota (ever) in anyway shape or form. One person tried to sign up twice, and I don’t send out mass emails. I talked to support in chat to contact dreamhost security, which I did. Nothing came back, not even confirmation. Any help, from dreamhost would be appreciated. ~D

Final-Recipient: rfc822;
Action: failed
Status: 5.0.0
Diagnostic-Code: X-Postfix; host[] said: 554
5.7.1 Recipient address rejected: Policy Rejection-
–SENDER_QUOTA_REJECTION-- (in reply to RCPT TO command)

This happened to me once and it was due to a glitch at the DreamHost end which they quickly cleared up. I suggest you file a Support ticket so they can track it down.