Hi There, I've never seen the error "Admin Prohibition" but the 550 states that the e-mail address does not exist. Try creating a domain debug log for that specific domain, you will get more detailed information regarding the connection, and be able to see the full SMTP conversation.
System Administration > Log Subscription > Add Log Subscription
from there make sure you choose "Domain Debug Log" as the log type and then specify the domain. Commit changes and then tail in the CLI.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...