Yes, some people forward to your service from other providers.
Yes, sometimes this content is probably spam...
If you send 4.7.0, and you mean to reject the message, please give a 5.x.x.
If you give a temp fail message, it gives the server the impression you intend
to attempt delivery later.
... 421-4.7.0 ...best protect our users from spam, the message has been blocked. ...
4.XXX.YYY Persistent Transient Failure A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future may be successful.
5.x.x is probably what you mean:
5.XXX.YYY Permanent Failure A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery.
Update - Seems you do get it (even if it does take a bit)...
Mar 3 00:36:27 ... submission Mar 3 00:36:28 421-4.7.0 - gsmtp ... Mar 3 00:42:57 421-4.7.0 - gsmtp Mar 3 00:43:02 421-4.7.0 - gsmtp ... Mar 3 01:12:45 421-4.7.0 - gsmtp Mar 3 01:12:45 550-5.7.1 - gsmtp