Where is the problem? RoundCube delivers the message to your SMTP server. SMTP server sends the message and gets error response 451, which means that the other side has Greylisting (
Greylisting - Wikipedia, the free encyclopedia) enabled. Now your SMTP server should queue the message for resending (f.e. 5 minutes later).
I see nothing where RoundCube is involved.
P.S.: Hmmm, does the baviour you described change, if you set $rcmail_config['smtp_helo_host'] accordingly to smtp server host name?