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.