How Sendmail Works
As stated before, sendmail can handle both incoming and outgoing mail for your domain. Take a closer look.
Usually each user in your home has a regular Linux account on your mail server. Mail sent to each of these users (firstname.lastname@example.org) eventually arrives at your mail server and sendmail then processes it and deposits it in the mailbox file of the user's Linux account.
Mail isn't actually sent directly to the user's PC. Users retrieve their mail from the mail server using client software, such as Microsoft's Outlook or Outlook Express, that supports either the POP or IMAP mail retrieval protocols.
Linux users logged into the mail server can read their mail directly using a text-based client, such as mail, or a GUI client, such as Evolution. Linux workstation users can use the same programs to access their mail remotely.