The Mail Target Agent (MTA) log contains the error:
"CSession::Logon Error -2147221219 The Microsoft Exchange Server computer is not available. Either there are network problems or the Microsoft Exchange Server computer is down for maintenance. - MAPI_E_FAILONEPROVIDER (Microsoft Exchange Server Information Store) Low level error: 0x0 File: 'aeWrapHelpers.h' Line '279' "
The target Exchange 2007 server does not have a Public Folder store as required.
Please create a Public Folder store in the Target Exchange 2007 organization:
Once the database has been created, stop and restart the MSExchangeIS service.
Please also see solution SOL44851 for information on the requirement for the Public Folder stores in Exchange 2007:
"Migration to Exchange 2007 - what is the requirement regarding Public Folders?"