501 Connection rejected by policy [7.7] xxxxxx, please visit www.messagelabs.com/support for more details about this error message.
Customers seeing this message are probably listed in Messagelabs internal RBL. Unfortunately Messagelabs will only discuss the block with their own customers, meaning you need to contact the person/company you were trying to email by phone or alternate means. Ask them to open a ticket with Messagelabs by emailing firstname.lastname@example.org. In that support request they either need to ask Messagelabs to remove your mail server's IP address from their RBL, or ask Messagelabs to talk with you directly regarding the issue.