Adding 3rd party components, how can I keep my code flexible enough so that I could replace component down the road

Adding 3rd party components
 
I was wondering how I could add 3rd party components without hard coding them into my code.
 
Let say I buy an email component and add the dll to my web application. How can I make sure I write my code flexible enough so that down the road I could replace the  email component with a different one without having to rewrite my whole code. Any advice would be greatly appreciated.

 
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