Red Hat Enterprise Linux 5 Administration Unleashed

368CHAPTER 18Setting Up an Email Server with SendmailSender’s MUAIMAP ServerInternetMTA ServerRecipient’s MUAFIGURE 18.1Sending an EmailAs shown in Figure 18.1, after the message is formatted and optionally encrypted, it issent to a Mail Transport Agent (MTA) using the Simple Mail Transfer Protocol (SMTP). Theemail application used is configured to send all outgoing emails to a specific MTA server.After that, the email may be transferred to several MTA servers before reaching the MTAfor the domain of the email recipient. After the email reaches its destination MTA, it isstored on the server and waits for the recipient to retrieve it.The email transfer in Figure 18.1 shows the initial MTA inside the same private networkas the device used to send the email. However, the MTA doesn’t have to be inside theprivate network. It only needs to be accessible by the system sending the email. Forexample, home users are given an MTA server (sometimes called an SMTP server) to use,which is accessible over the Internet.If you are administering systems for a company that transfers a large amount of email aswell as company confidential email, you will need to set up an MTA inside the company’sprivate network. As shown in Figure 18.2, if an employee sends an email to anotheremployee inside the private network with an MTA server, the email goes through the privateMTA server and is never transmitted across the Internet, thus making it harder for someoneoutside the company to intercept and read the email containing confidential information.