Die Fehlermeldung kommt IMMER vom Empfänger.
4.X.X Persistent Transient Failure
A persistent transient failure is one in which the message as
sent is valid, but some temporary event prevents the successful
sending of the message. Sending in the future may be successful.
X.5.X Mail Delivery Protocol Status
The mail delivery protocol status codes report failures
involving the message delivery protocol. These failures
include the full range of problems resulting from
implementation errors or an unreliable connection. Mail
delivery protocol issues may be controlled by many parties
including the originating system, destination system, or
intermediate system administrators.
X.5.1 Invalid command
A mail transaction protocol command was issued which was
either out of sequence or unsupported. This is useful only
as a permanent error.
Enhanced Mail System Status Codes (http://www.ietf.org/rfc/rfc1893.txt)
Entweder haben die beim Empfänger den Server paranoid oder falsch konfiguriert. Mach doch mal händisch eine SMTP-Session auf und schau, was dabei zurück kommt.