I want to add reusability and removability. If I for example write some code that connects to a SMTP server to send an e-mail, I probably will re-use it in othe

I want to add reusability and removability. If I for example write some code that connects to a SMTP server to send an e-mail, I probably will re-use it in other places, even in other programs, so the code can't depend on global functions. The code should be independent. Then it also becomes removable and replaceable.