A rough example of how SMTP works
Imagine the Server being the address and the Port being the apartment number. To send an email, microletter connects to the SMTP Server. In this example, this connection will be a postman. The postman searches for the building (server) and apartment (port) to find the right door. Before the postman can hand over the letter, he needs to authenticate himself. For that, he uses the Username and Password. If that all worked correctly, the letter (aka. the email) can be delivered.