Enable and Configure Protocol Logging
http://technet.microsoft.com/en-us/library/bb124531.aspx
Potocol logging records the SMTP conversations that occur between e-mail servers as part of message delivery. These SMTP conversations occur on Send connectors and Receive connectors configured on servers running Microsoft Exchange Server 2010 that have the Hub Transport server role or the Edge Transport server role installed. You can use protocol logging to diagnose mail flow problems.
By default, protocol logging is disabled on all Send connectors and Receive connectors. Protocol logging is enabled or disabled on a per-connector basis. Other protocol logging options are set on a per-connector type basis for the whole server. All the Receive connectors on a Hub Transport server or an Edge Transport server share the same protocol log files and protocol log options. These protocol log files and protocol log options are separate from the Send connector protocol log files and protocol log options that are on the same server. By default, Exchange 2010 uses circular logging to limit the protocol logs based on file size and file age to help control the hard disk space used by the protocol log files.