Delays delivering to recipients protected by Proofpoint
Mailprotector is actively working towards an open dialog with Proofpoint to resolve the strange behavior.
If you have received a report from your client(s) regarding a delay or bounce and the NDR or logs show an SMTP response of:
450 4.7.9 Deferred - see https://support.proofpoint.com/dnsbl-lookup.cgi?ip=x.x.x.x
That is a Proofpoint Dynamic Reputation List block.
Mailprotector is proactively monitoring the logs for instances of this situation and mitigating the use of the outbound IP address that is affected. You may still receive a report from a client because an email must trigger this notice, but rest assured, Mailprotector is taking every action we can to minimize the delay of emails to Proofpoint.