Some nice sources of Exchange 2007 Information:

Microsoft documentation
http://www.microsoft.com/technet/prodtechnol/exchange/2007/downloads/documentation.mspx

Microsoft Exchange Server 2007 Transport Server Role Architecture Diagrams
http://www.microsoft.com/downloads/details.aspx?familyid=612F811D-2953-4C08-945E-833C17150083&displaylang=en

Exchange Management Shell Quick Reference
http://www.microsoft.com/downloads/details.aspx?familyid=01A441B9-4099-4C0F-B8E0-0831D4A2CA86&displaylang=en

Exchange 2007 Transport Permissions Model
http://www.microsoft.com/technet/prodtechnol/exchange/E2k7Help/315d9c42-1ab4-4ef4-9292-12cdcb9c98cf.mspx?mfr=true

Receive Connectors
http://www.microsoft.com/technet/prodtechnol/exchange/e2k7help/17751a60-39fe-433f-84d2-bfc14ff4ba51.mspx?mfr=true

Allowing servers to relay off EXchange 2007 (Sharepoint, IIS, etc.)
http://msexchangeteam.com/archive/2006/12/28/432013.aspx

Install antispam agents on single Hub Transport server - with no edge transport
http://blog.justinho.com/PermaLink,guid,bb75cfa8-b76a-48e7-afed-d593693dc135.aspx