In general the error indicates the recipient server we are attempting to connect to is refusing the connection for some reason, whether because the server is down or it is greylisting the IP. We would need to know more specifics though to determine exactly what is going on. A situation like this, I highly recommend contacting the Support Team that works with your account for additional troubleshooting.
Just wanted to let you all know that I had the same problem with a local 2010 Exchange Server and McAfee. I got it all to work today as I created a different type of receive connector in Exchange. McAfee gives you a list of the CIDR's to add as IP/21 or IP/24, but when I put these on the server's receive connector I could not get it to work and I got the error above."Message Disposition: [451 Could not verify recipients" Instead of entering these in the receive connector I choose the option to add IP and SubMask on the Exchange server for the receive connector. Enter the IP and then the Subnet Mask EX:188.8.131.52 and SubMask: 255.255.248.0. I also created a new Receive connector on the Exchange server and added only these IP's on Port 25. I then move MX records and email is now flowing.
Again all I did was create a different type of connector with IP and Mask and then created new Receive connector. All is working for me now!