This is initially only a hint that the message could not yet be transmitted successfully.
The SMTP status code 450 4.3.2 indicates that the Exchange Server of the other side has frozen the reception of messages, e.g. because a problem is being solved there.

“This notification can be ignored if you don’t receive another tracker that reports an error for the message in question.

This means that this is only a warning; if the message cannot be delivered repeatedly, you will receive an error message.
e.g: companies do not need to do anything at this point, eGate simply informs you that the message is not yet available on the other side.