1 00:00:00,006 --> 00:00:02,000 - [Instructor] Email's an essential part 2 00:00:02,000 --> 00:00:04,003 of virtually every organization. 3 00:00:04,003 --> 00:00:06,009 Let's first discuss mail retrieval protocols, 4 00:00:06,009 --> 00:00:10,009 POP and IMAP, then cover mail transmission with SMTP. 5 00:00:10,009 --> 00:00:14,008 The Post Office Protocol is a legacy email retrieval system. 6 00:00:14,008 --> 00:00:16,006 The most current version is three, 7 00:00:16,006 --> 00:00:20,000 which is why it is most often referred to as POP3. 8 00:00:20,000 --> 00:00:24,000 A POP3 server listens on TCP port 110. 9 00:00:24,000 --> 00:00:26,007 An email client will connect to a POP3 server, 10 00:00:26,007 --> 00:00:30,009 download any new emails, store them on the client's PC, 11 00:00:30,009 --> 00:00:33,003 then delete them from the server. 12 00:00:33,003 --> 00:00:36,002 Most clients have the ability to leave a copy on the server, 13 00:00:36,002 --> 00:00:38,006 but this isn't the default behavior. 14 00:00:38,006 --> 00:00:40,007 POP3 does have a secure option 15 00:00:40,007 --> 00:00:45,000 whereby communication can be encrypted using TLS or SSL. 16 00:00:45,000 --> 00:00:49,000 POP3S runs on TCP port 995. 17 00:00:49,000 --> 00:00:50,009 There are several authentication methods 18 00:00:50,009 --> 00:00:53,004 supported by the POP3 extension mechanisms, 19 00:00:53,004 --> 00:00:59,009 such as SASL, a Kerberos version and APOP that uses MD5. 20 00:00:59,009 --> 00:01:02,006 Due to its advanced features and flexibility, 21 00:01:02,006 --> 00:01:04,009 the Internet Message Access Protocol 22 00:01:04,009 --> 00:01:08,002 has supplanted POP3 for mail retrieval. 23 00:01:08,002 --> 00:01:10,000 IMAP was designed with the idea 24 00:01:10,000 --> 00:01:13,009 that a user will access their email from multiple clients. 25 00:01:13,009 --> 00:01:15,008 It's for this reason that by default, 26 00:01:15,008 --> 00:01:17,007 email is left on the server. 27 00:01:17,007 --> 00:01:21,009 Standard IMAP is available via TCP port 143, 28 00:01:21,009 --> 00:01:25,000 where the secure version IMAPS is available 29 00:01:25,000 --> 00:01:27,008 via TCP port 993. 30 00:01:27,008 --> 00:01:30,001 IMAP tends to stay connected to the server. 31 00:01:30,001 --> 00:01:33,004 So as messages arrive, they're quickly delivered. 32 00:01:33,004 --> 00:01:35,007 POP allows only a single client to access 33 00:01:35,007 --> 00:01:38,007 a mailbox at a time where IMAP specifically 34 00:01:38,007 --> 00:01:42,008 allows multiple clients and simultaneous access. 35 00:01:42,008 --> 00:01:44,006 IMAP tracks message states also. 36 00:01:44,006 --> 00:01:49,005 So each email can be tagged as read, replied to, or deleted. 37 00:01:49,005 --> 00:01:51,007 These flags are stored server-sides 38 00:01:51,007 --> 00:01:54,009 so multiple clients can see the state of messages. 39 00:01:54,009 --> 00:01:57,007 IMAP supports server-side searches also. 40 00:01:57,007 --> 00:02:00,001 This allows clients to pass certain criteria 41 00:02:00,001 --> 00:02:01,008 to the server for searching. 42 00:02:01,008 --> 00:02:03,007 The Simple Mail Transfer Protocol 43 00:02:03,007 --> 00:02:06,002 was originally defined in 1983, 44 00:02:06,002 --> 00:02:11,007 but more recently revised in 2003 by RFC 5321. 45 00:02:11,007 --> 00:02:15,000 It defaults to TCP port 25. 46 00:02:15,000 --> 00:02:18,004 As the name implies, it is generally used by mail servers 47 00:02:18,004 --> 00:02:20,003 to both send and receive email 48 00:02:20,003 --> 00:02:22,004 from outside of the organization. 49 00:02:22,004 --> 00:02:24,009 An email client that uses POP or IMAP 50 00:02:24,009 --> 00:02:29,009 might also send using SMTP, but it's not very likely. 51 00:02:29,009 --> 00:02:33,005 Clients that use proprietary extensions like Outlook's MAPI 52 00:02:33,005 --> 00:02:36,002 don't generally use SMTP to send email 53 00:02:36,002 --> 00:02:40,002 though the server will still transfer email via this method. 54 00:02:40,002 --> 00:02:42,003 SMTP was originally developed 55 00:02:42,003 --> 00:02:45,001 to carry standard ASCII text, not files. 56 00:02:45,001 --> 00:02:48,000 So Multipurpose Internet Mail Extensions 57 00:02:48,000 --> 00:02:50,008 were created to support binary files. 58 00:02:50,008 --> 00:02:54,009 Some networks will employ an SMTP relay server. 59 00:02:54,009 --> 00:02:57,004 The concept is only this single server 60 00:02:57,004 --> 00:03:01,002 is allowed to send SMTP message outbound from the network. 61 00:03:01,002 --> 00:03:05,003 This is usually used to act as a spam protection mechanism. 62 00:03:05,003 --> 00:03:09,003 Spam bots are often thwarted by only allowing SMTP 63 00:03:09,003 --> 00:03:11,004 to be sent from a relay server. 64 00:03:11,004 --> 00:03:14,008 SMTP relays often employs some basic mail scanning 65 00:03:14,008 --> 00:03:17,000 to further prevent spam. 66 00:03:17,000 --> 00:03:20,002 Believe it or not, basic SMTP functionality 67 00:03:20,002 --> 00:03:21,009 can be easy to test. 68 00:03:21,009 --> 00:03:25,002 Using Telnet, you can connect to an SMTP server 69 00:03:25,002 --> 00:03:28,005 and actually send an email via plain text commands. 70 00:03:28,005 --> 00:03:31,002 Since email has become so necessary, 71 00:03:31,002 --> 00:03:34,000 this topic could not be more important.