SMTP Connectors are the mechanism Exchange 2010 uses to transfer mail between the Exchange infrastructure and external systems using the SMTP protocol. These external systems can include other email domains on the Internet, smart hosts such as mail filtering appliances, your ISP’s smart host, your SharePoint farm, etc. They are found on Exchange servers running the Hub Transport and Edge Transport servers, and there are two kinds:
Planning an Exchange 2010 infrastructure properly can take weeks, and involve tons of information gathering, utilization forecasting, capacity planning, and differential calculus. Unfortunately, you don’t always have enough time to devote to planning our servers the way they ought to be, so we resort to SWAGs. While not as good as a detailed architectural review, this Exchange 2010 Sizing Cheat Sheet should get you closer to optimum than any SWAG will.