Install Exchange 2010 SP1!
Installing the service pack fixes the round-robin selection process for remote hub transport servers in other sites (see Hub Transport Load Balancing) so that only the IP addresses of operational servers are used.
Exchange 2010 runs on Windows 2008 (or 2008 R2) and this operating system supports IPv4 and IPv6. In fact it will provide an IPv6 address even if you don’t have an IPv6 router or infrastructure. But if a remote hub registers an IPv6 address in DNS then you might attempt to use the IPv6 address before any IPv4 address and fail to connect. Exchange 2010 SP1 will now remove the IPv6 address from the round-robin hub selection list, and so speed up transport in Exchange.