This event may occur if either of the following conditions is true:
1. There are general performance issues on either the computer that is running the Microsoft® Exchange Server 2007Hub Transport server role or the computer that is running the Microsoft Exchange Server 2007 Edge Transport server role.
2. There are issues with the transport agent itself.
Also, if you could disable the Firewall/ any Antivirus/spamfilter and try. I am sure it could be somehow to do with antivirus/spamfiltering..
Let us know the status.
Regards,
Vikas