Understanding POP3 and IMAP Protocols for Mail Handling

1. Created by : Ashish Shah, J. M. Patel College of Commerce Understanding POP3 / IMAP Unit – V Chap - 2

2. POP3 was developed to solve the problem of what happens to messages when the recipient is not connected to the network. POP3 runs on a server that is connected to a network and that continuously sends and receives mail. The POP3 server stores any messages it receives until the message recipients request them. Created by : Ashish Shah, J. M. Patel College of Commerce Understanding POP3

3. Without POP3 (or IMAP), and MUA and MTAcan’t communicate. That is, if you want to read your mail, you need POP3 (or IMAP) to retrieve messages that arrive at your MTA while you’re offline. POP3 uses the MTA’s storage to hold messages until they are requested. When users want to check their email, they connect to the POP3 server and retrieve messages that were stored by the server. Created by : Ashish Shah, J. M. Patel College of Commerce Understanding POP3

4. After retrieval, the messages are stored locally (that is, locally to the MUA) and you can use the MUA on your PC to read them at your side. Of course, your MUA has to understand the POP3 to be able to communicate with the POP3 server, but most MUAs speak fluent POP3 these days so this is rarely a problem. The messages you retrieve to your PC are then typically removed from the server. Created by : Ashish Shah, J. M. Patel College of Commerce Understanding POP3

5. The Internet Message Access Protocol version 4 (IMAP4) provides much more sophisticated email-handling functionality than SMTP or POP3 do. IMAP4 has more features. IMAP4 enables you to store email on a networked mail server, just as POP3 does. The difference is that POP3 requires you to download your email before your MUA reads it, whereas IMAP4 enables your email to reside permanently on a remote server, from which you can access your mail. Created by : Ashish Shah, J. M. Patel College of Commerce Understanding IMAP4

6. you can do so from your office, your home, your PDA, your cell phone, or anywhere else. Your MUA must understand IMAP4 to retrieve messages from an IMAP4 server. Created by : Ashish Shah, J. M. Patel College of Commerce

7. Before you begin configuring Sendmail, be sure it’s installed. It probably is, because the installation program installs Sendmail. But just to be sure, check it out. The following example shows how to check using the rpmquery command. The output shows not only that Sendmail is installed, but which version of Sendmail is installed: Created by : Ashish Shah, J. M. Patel College of Commerce Configuring Sendmail

8. # rpmquery -a | grep sendmail sendmail-cf -8.13.4-1.1 sendmail-doc-8.13.4-1.1 sendmail-devel-8.13.4-1.1 sendmail-8.13..4-1.1 Created by : Ashish Shah, J. M. Patel College of Commerce Configuring Sendmail

9. If Sendmail is installed, make sure that it starts at boot time. You can use the following chkconfig command to verify this: # chkconfig --list sendmail sendmail 0:off 1:off 2:on 3:on 4:on 5:on 6:off In the example, the sendmail service, which controls the Sendmail MTA, will start in run levels 2, 3, 4, and 5 and stop in run levels 0, 1, and 6. If you don’t see this type of output, use the following chkconfig command to correct it: # chkconfig --levels 0123456 sendmail off # chkconfig --levels 2345 sendmail on Created by : Ashish Shah, J. M. Patel College of Commerce Configuring Sendmail

10. The following command uses ps and pidof to query the process table for running Sendmail instances: # ps uw -p $(pidof sendmail) USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND root 3706 0.0 0.6 8652 3292 ? Ss 22:45 0:00 sendmail: accepting connections smmsp 3714 0.0 0.5 7704 2580 ? Ss 22:45 0:00 sendmail: Queue r unner@01:00:00 for /var/spool/clientmqueue Created by : Ashish Shah, J. M. Patel College of Commerce Configuring Sendmail

11. Notice that the TTY fields are ?, indicating that the sendmail processes are not associated with a terminal. This usually means that the process is running as daemon, which is the case with Sendmail. If you don’t see output indicating that Sendmail is running, execute the following command to start Sendmail: # service sendmail start Starting sendmail: [ OK ] Starting sm-client: [ OK ] Created by : Ashish Shah, J. M. Patel College of Commerce Configuring Sendmail

12. Another technique you can use is to telnet to localhost, port 25. If Sendmail is running, you’ll see something resembling the following: # telnet localhost 25 Trying 127.0.0.1... Connected to localhost.localdomain (127.0.0.1). Escape character is ‘^]’. 220 bookbeast.example.com ESMTP Sendmail 8.13.1/8.13.1; Sat, 12 Mar 2005 22:51: 43 -0500 QUIT Created by : Ashish Shah, J. M. Patel College of Commerce

13. Type QUIT to exit the session. This output means that Sendmail is running and responding to incoming SMTP sessions. Created by : Ashish Shah, J. M. Patel College of Commerce

14. Many system administrators think that Sendmail is difficult to configure. If you look at its configuration file, /etc/mail/sendmail.cf, this might seem to be the case. The following example specifies the host named mailbeast.example.com as a mail relay host. # “Smart” relay host (may be null) Created by : Ashish Shah, J. M. Patel College of Commerce Configuring Sendmail

15. For all but the most unusual or extreme mail system configurations, all you need is a copy of the predefined configuration files in /etc/mail, sendmail.mc The basic procedure is to modify sendmail.mc for your own needs, regenerate /etc/mail/sendmail.cf using the m4 macro processor, as explained in a moment, and then test your configuration. Fedora Core and RHEL ship with a generic Sendmail configuration file (/etc/mail/sendmail.cf). Created by : Ashish Shah, J. M. Patel College of Commerce Configuring Sendmail