Please note that this is an expected behaviour of the Messaging Gateway.
The reasons are as follows:
- An SMTP 5XX response is based on a permanent condition, which may involve the sender, recipient, IP address, message content, or other reasons.
- An SMTP 4XX response is temporary, which may indicate the server too busy, unable to validate something, etc.
For all practical purposes, 5XX responses are based on the client message transfer agent or MTA, which doesn't change, and trying a different (presumably identically configured) MTA will have the same result.
A 4XX response is based on the server, and hopefully a different server will not have the same (temporary) problem, thus being able to accept the message.
Furthermore, many 554 responses are generally the result of either a prohibited IP address, or some sort of blocked content in the message itself.