Can’t send email to juno addresses

Recently we are having a problem sending email to anyone with a juno.com address. We have always been able to send to them until the last few weeks. We have Exchange Server 5.5 SP3 and Groupshield 4.5. There have been no changes to this server recently. Here is the message that is returned when we try to send email:

The recipient was unavailable to take delivery of the message. 550 Access denied

Anyone have any ideas?