Recently one of our clients upgraded to Virgins new 50mb Service for there existing ADSL2+ Service. The day of change over we moved IP's and the server started to accept new email from the new IP, accept SMTP transmissions were timing out. We adding a middleman SMTP server inbetween the Spam Filter and the Exchange box with a low MTU connection size 1440 and this fixed the issue temporarily.  After a week plus with Virgin Technical Support, Virgin have found there is a problem with their new 50Mb service. There’s a workaround which is to reduce the MTU to 1460 on the receiving server

HKLM\System\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\[ID of interface]\MTU

The virgin router partners Netgear are working on a firmware update for their new superhubs to fix it properly.