Imagine that, at the end of your driveway, you had two mailboxes. One was only for the mail that you sent out, and the other was only for the mail that you received. That’s pretty much how email works. Most email accounts have two servers: one that lets you send emails to other people, and another that lets you receive the emails that other people send you.
The server that lets you send mail is called an outgoing, or SMTP server. The server that lets you receive mail is called an incoming, POP, or just Mail server. These servers will have slightly different addresses. For example, the outgoing mail server for Yahoo mail is named smtp.mail.yahoo.com. The incoming mail server for Yahoo mail is named pop.mail.yahoo.com. Usually, both servers will use the same username and password for your account.
If you try to send outgoing mail through the incoming mail server, it will not work. If you try to receive incoming mail through the outgoing mail server, then it will not work. If you’re having problems sending or receiving mail through your service, please check their Help web pages for your service, or contact your email service provider.