Microsoft TechNet - Exchange 2010 - Understanding Transport Agents
MSDN - Tell Those Messages Where to Go By Using Exchange 2007 SP1 Transport Agents
Exchange Sender Based Routing
The Exchange Team Blog - Sample Exchange 2007 transport agent - add the name of the group to subject line
Corelan Team - Free tool - Attachment filtering with Exchange 2007/2010 (custom transport agent)
Corelan Team - Free tool - Exchange 2007/2010 : Renaming attachments ‘on the fly’ – custom transport agent
GitHub - Pro/dkim-exchange DKIM Signing Agent for Microsoft Exchange Server
The Exchange Team Blog - How to write Exchange 2007 transport agents
The Exchange Team Blog - How to write an Exchange 2013 transport agent
Office development > Exchange Server > Exchange Online and Exchange 2013 development > Transport agents > Creating transport agents
MSDN Blogs > mstehle: The CDOs and CDONTS of Messaging Development > HOWTO: Sample Transport Agent – Add Headers, Categories, MAPI Props, Even Uses a Fork!