The document describes the Simple Mail Transfer Protocol (SMTP) which is used for sending and receiving email. It outlines the key components of SMTP including Mail Transfer Agents (MTAs), Mail Delivery Agents (MDAs), and the core SMTP commands used to send mail such as HELO, MAIL FROM, RCPT TO, and DATA. It also provides examples of using the telnet command line tool to interact with an SMTP server and send a basic email.

1. Send Mail TransportSend Mail Transport
Protocol (SMTP)Protocol (SMTP)
➢ 1982 - RFC 8211982 - RFC 821
➢ 2008 - RFC 5321 (ESMTP)2008 - RFC 5321 (ESMTP)
➢ TCP ports 25 and 587TCP ports 25 and 587
➢ Mail User Agent(MUA)Mail User Agent(MUA)
➢ Mail Submission Agent(MSA)Mail Submission Agent(MSA)
➢ Mail Transfer Agent(MTA)Mail Transfer Agent(MTA)
➢ Mail Delivery Agent(MDA)Mail Delivery Agent(MDA)

2. MUA
MSAMSA
MTAMTA
MDAMDA
MTAMTA
mailmail
queuequeue
-mail1-mail1
-mail2-mail2
-mail3-mail3
mailboxmailbox

3. SMTP commandsSMTP commands
➢ HELOHELO
➢ EHLOEHLO
➢ MAIL FROM:MAIL FROM:
➢ RCPT TO:RCPT TO:
➢ DATADATA
➢ RSETRSET
➢ VRFYVRFY
➢ HELPHELP
➢ QUITQUIT
➢ Valid replies (2xx)Valid replies (2xx)
➢ Transient errors(4xx)Transient errors(4xx)
➢ Permanent errors(5xx)Permanent errors(5xx)

4. SMTP commandsSMTP commands
hackman@terion:~$ telnet yuhu.biz 25hackman@terion:~$ telnet yuhu.biz 25
Trying 85.14.7.4...Trying 85.14.7.4...
Connected to yuhu.biz.Connected to yuhu.biz.
Escape character is '^]'.Escape character is '^]'.
220 blackpearl.yuhu.biz ESMTP Postfix (2.1.1)220 blackpearl.yuhu.biz ESMTP Postfix (2.1.1)
HELOHELO
501 Syntax: HELO hostname501 Syntax: HELO hostname
HELO yuhu.bizHELO yuhu.biz
250 blackpearl.yuhu.biz250 blackpearl.yuhu.biz

5. SMTP commandsSMTP commands
hackman@terion:~$ telnet yuhu.biz 25hackman@terion:~$ telnet yuhu.biz 25
Trying 85.14.7.4...Trying 85.14.7.4...
Connected to yuhu.biz.Connected to yuhu.biz.
Escape character is '^]'.Escape character is '^]'.
220 blackpearl.yuhu.biz ESMTP Postfix (2.1.1)220 blackpearl.yuhu.biz ESMTP Postfix (2.1.1)
EHLO yuhu.bizEHLO yuhu.biz
250-blackpearl.yuhu.biz250-blackpearl.yuhu.biz
250-PIPELINING250-PIPELINING
250-SIZE 30720000250-SIZE 30720000
250-VRFY250-VRFY
250-ETRN250-ETRN
250-STARTTLS250-STARTTLS
250-AUTH CRAM-MD5 PLAIN LOGIN DIGEST-MD5250-AUTH CRAM-MD5 PLAIN LOGIN DIGEST-MD5
250-AUTH=CRAM-MD5 PLAIN LOGIN DIGEST-MD5250-AUTH=CRAM-MD5 PLAIN LOGIN DIGEST-MD5
250 8BITMIME250 8BITMIME

6. SMTP commandsSMTP commands
hackman@terion:~$ telnet yuhu.biz 25hackman@terion:~$ telnet yuhu.biz 25
Trying 85.14.7.4...Trying 85.14.7.4...
Connected to yuhu.biz.Connected to yuhu.biz.
Escape character is '^]'.Escape character is '^]'.
HELO yuhu.bizHELO yuhu.biz
250 blackpearl.yuhu.biz250 blackpearl.yuhu.biz
MAIL FROM: mm@yuhu.bizMAIL FROM: mm@yuhu.biz
250 Ok250 Ok
RCPT TO: mm@yuhu.bizRCPT TO: mm@yuhu.biz
250 Ok250 Ok
DATADATA
354 End data with <CR><LF>.<CR><LF>354 End data with <CR><LF>.<CR><LF>
Subject: testingSubject: testing
test 1test 1
..
250 Ok: queued as 450D1156263C250 Ok: queued as 450D1156263C

7. SMTP commandsSMTP commands
hackman@terion:~$ telnet yuhu.biz 25hackman@terion:~$ telnet yuhu.biz 25
Trying 85.14.7.4...Trying 85.14.7.4...
Connected to yuhu.biz.Connected to yuhu.biz.
Escape character is '^]'.Escape character is '^]'.
HELO yuhu.bizHELO yuhu.biz
250 blackpearl.yuhu.biz250 blackpearl.yuhu.biz
VRFY hackman@yuhu.bizVRFY hackman@yuhu.biz
252 hackman@yuhu.biz252 hackman@yuhu.biz
VRFY dsadas@yuhu.bizVRFY dsadas@yuhu.biz
550 <dsadas@yuhu.biz>: Recipient address550 <dsadas@yuhu.biz>: Recipient address
rejected: User unknown in virtual mailbox tablerejected: User unknown in virtual mailbox table
RSETRSET
250 Ok250 Ok
quitquit
221 Bye221 Bye

8. SMTPSMTP
➢ Retry interval, at least 30minRetry interval, at least 30min
➢ Give-up time, at least 4-5 daysGive-up time, at least 4-5 days
➢ Storage - either Mailbox or MaildirStorage - either Mailbox or Maildir
➢ Failover setupFailover setup

9. SMTPSMTP
Mailbox vs. MaildirMailbox vs. Maildir
mail# ls -1Amail# ls -1A
SpamSpam
mail-trashmail-trash
saved-draftssaved-drafts
saved-messagessaved-messages
sent-mailsent-mail
mail# ls -1Amail# ls -1A
camera/camera/
cur/cur/
new/new/
tmp/tmp/
mail# ls -1Amail# ls -1A
SpamSpam
mail-trashmail-trash
saved-draftssaved-drafts
saved-messagessaved-messages
sent-mailsent-mail

10. SMTP failoverSMTP failover
IN MX 10 mail.example.com.IN MX 10 mail.example.com.
IN MX 20 mail2.example.com.IN MX 20 mail2.example.com.
mailmail IN A 123.123.13.11IN A 123.123.13.11
mail2mail2 IN A 123.123.13.12IN A 123.123.13.12
.13.11.13.11
.13.12.13.12

11. SMTP failoverSMTP failover
IN MX 10 mail.example.com.IN MX 10 mail.example.com.
IN MX 20 mail2.example.com.IN MX 20 mail2.example.com.
mailmail IN A 123.123.13.11IN A 123.123.13.11
mail2mail2 IN A 123.123.13.12IN A 123.123.13.12
.13.11.13.11
.13.12.13.12

12. SMTP failoverSMTP failover
IN MX 10 mail.example.com.IN MX 10 mail.example.com.
IN MX 20 mail2.example.com.IN MX 20 mail2.example.com.
mailmail IN A 123.123.13.11IN A 123.123.13.11
mail2mail2 IN A 123.123.13.12IN A 123.123.13.12
.13.11.13.11
.13.12.13.12

13. Post Office Protocol - POPPost Office Protocol - POP
➢ 1984 – POP1984 – POP - RFC 918- RFC 918
➢ 1985 – POP21985 – POP2 - RFC 973- RFC 973
➢ 1988 – POP31988 – POP3 - RFC 1081- RFC 1081
➢ 1996 – POP31996 – POP3 - RFC 1939- RFC 1939
➢ TCP ports 110, 995(SSL)TCP ports 110, 995(SSL)

14. POP3 commandsPOP3 commands
➢ USERUSER
➢ PASSPASS
➢ LISTLIST
➢ STATSTAT
➢ UIDLUIDL
➢ RETRRETR
➢ DELEDELE
➢ QUITQUIT
➢ Valid replies (+OK)Valid replies (+OK)
➢ Negative replies(-ERR)Negative replies(-ERR)
➢ Single connectionSingle connection

15. POP3 commandsPOP3 commands
hackman@BlackPearl: ~$ telnet localhost 110hackman@BlackPearl: ~$ telnet localhost 110
Trying 127.0.0.1...Trying 127.0.0.1...
Connected to 127.0.0.1.Connected to 127.0.0.1.
Escape character is '^]'.Escape character is '^]'.
+OK+OK Dovecot ready.Dovecot ready.
USER userX@yuhu.bizUSER userX@yuhu.biz
+OK+OK
PASS Jer0n1m0!PASS Jer0n1m0!
+OK Logged in.+OK Logged in.
STATSTAT
+OK 1717 21744442+OK 1717 21744442

16. POP3 commandsPOP3 commands
LISTLIST
+OK 1717 messages:+OK 1717 messages:
1 84821 8482
2 808532 80853
3 330933 33093
4 55434 5543
..........
1715 80601715 8060
1716 65581716 6558
1717 46151717 4615
..
RETR 1717RETR 1717
___mail message here______mail message here___
..
DELE 1717DELE 1717
+OK Marked to be deleted.+OK Marked to be deleted.

17. Interactive Message AccessInteractive Message Access
Protocol - IMAPProtocol - IMAP
➢ 1988 – IMAP 21988 – IMAP 2 - RFC 1064- RFC 1064
➢ 1991 – IMAP 31991 – IMAP 3 - RFC 1203- RFC 1203
➢ 2003 – IMAP 42003 – IMAP 4 - RFC 3501- RFC 3501
➢ becomes INTERNET MESSAGE ACCESS PROTOCOLbecomes INTERNET MESSAGE ACCESS PROTOCOL
➢ 2011 – SRV records – RFC 61862011 – SRV records – RFC 6186
➢ 2013 – IMAP 42013 – IMAP 4 - RFC 6858- RFC 6858
➢ TCP port 143, 993(SSL)TCP port 143, 993(SSL)

18. Interactive Message AccessInteractive Message Access
Protocol - IMAPProtocol - IMAP
➢ Support multiple connectionsSupport multiple connections
➢ Supports foldersSupports folders
➢ Support pulling only the headers ofSupport pulling only the headers of
the e-mail, pull the data on demandthe e-mail, pull the data on demand
➢ Support mail PUSH(sending mail)Support mail PUSH(sending mail)

19. IMAP SRV records - RFC6186IMAP SRV records - RFC6186
_submission._tcp.example.com. SRV 0 1 587_submission._tcp.example.com. SRV 0 1 587
mail.example.com.mail.example.com.
_service._proto.name TTL class SRV priority weight port_service._proto.name TTL class SRV priority weight port
targettarget
- service: the symbolic name of the desired service.- service: the symbolic name of the desired service.
- proto: the transport protocol of the desired service; this is- proto: the transport protocol of the desired service; this is
usually either TCP or UDP.usually either TCP or UDP.
- name: the domain name for which this record is valid.- name: the domain name for which this record is valid.
- TTL: standard DNS time to live field.- TTL: standard DNS time to live field.
- class: standard DNS class field (this is always IN).- class: standard DNS class field (this is always IN).
- priority: the priority of the target host, lower value means- priority: the priority of the target host, lower value means
more preferred.more preferred.
- weight: A relative weight for records with the same priority.- weight: A relative weight for records with the same priority.
- port: the TCP or UDP port on which the service is to be- port: the TCP or UDP port on which the service is to be
found.found.
- target: the canonical hostname of the machine providing- target: the canonical hostname of the machine providing
the service.the service.

20. IMAP Basic commandsIMAP Basic commands
➢ a001 login USER PASSa001 login USER PASS
➢ a001 logouta001 logout
➢ a001 select FOLDERa001 select FOLDER
➢ a001 list "" *a001 list "" *
➢ a001 fetch PARAMa001 fetch PARAM
➢ ALL/HEADERS/BODYALL/HEADERS/BODY
➢ a001 delete "Message"a001 delete "Message"
➢ Valid replies - (a00x OK)Valid replies - (a00x OK)
➢ Negative replies - (a00x BAD or NO)Negative replies - (a00x BAD or NO)

21. IMAPIMAP
hackman@BlackPearl: ~$ telnet localhost 143hackman@BlackPearl: ~$ telnet localhost 143
Trying 127.0.0.1...Trying 127.0.0.1...
Connected to 127.0.0.1.Connected to 127.0.0.1.
Escape character is '^]'.Escape character is '^]'.
* OK* OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR[CAPABILITY IMAP4rev1 LITERAL+ SASL-IR
LOGIN-REFERRALS ID ENABLE IDLE STARTTLSLOGIN-REFERRALS ID ENABLE IDLE STARTTLS
AUTH=PLAIN] Dovecot ready.AUTH=PLAIN] Dovecot ready.
a001 login userX@yuhu.biz Jer0n1m0!a001 login userX@yuhu.biz Jer0n1m0!
a001 OKa001 OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR[CAPABILITY IMAP4rev1 LITERAL+ SASL-IR
LOGIN-REFERRALS ID ENABLE IDLE SORTLOGIN-REFERRALS ID ENABLE IDLE SORT
SORT=DISPLAY THREAD=REFERENCESSORT=DISPLAY THREAD=REFERENCES
THREAD=REFS MULTIAPPEND UNSELECT CHILDRENTHREAD=REFS MULTIAPPEND UNSELECT CHILDREN
NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1
CONDSTORE QRESYNC ESEARCH ESORT SEARCHRESCONDSTORE QRESYNC ESEARCH ESORT SEARCHRES
WITHIN CONTEXT=SEARCH LIST-STATUS QUOTA]WITHIN CONTEXT=SEARCH LIST-STATUS QUOTA]
Logged inLogged in

22. IMAPIMAP
a002 lista002 list
a002 BADa002 BAD Error in IMAP command LIST: InvalidError in IMAP command LIST: Invalid
reference.reference.
a003 select inboxa003 select inbox
* FLAGS (Answered Flagged Deleted Seen Draft* FLAGS (Answered Flagged Deleted Seen Draft
$label2)$label2)
* OK [PERMANENTFLAGS (Answered Flagged* OK [PERMANENTFLAGS (Answered Flagged
Deleted Seen Draft $label2 *)] Flags permitted.Deleted Seen Draft $label2 *)] Flags permitted.
* 1714 EXISTS* 1714 EXISTS
* 0 RECENT* 0 RECENT
* OK [UIDVALIDITY 1330989345] UIDs valid* OK [UIDVALIDITY 1330989345] UIDs valid
* OK [UIDNEXT 59881] Predicted next UID* OK [UIDNEXT 59881] Predicted next UID
* OK [HIGHESTMODSEQ 1] Highest* OK [HIGHESTMODSEQ 1] Highest
a003 OKa003 OK [READ-WRITE] Select completed.[READ-WRITE] Select completed.

23. IMAPIMAP
a004 list "" *a004 list "" *
* LIST (HasNoChildren) "." "Spam"* LIST (HasNoChildren) "." "Spam"
* LIST (HasNoChildren) "." "INBOX"* LIST (HasNoChildren) "." "INBOX"
a004 OKa004 OK List completed.List completed.
a005 FETCH 1:15 ENVELOPEa005 FETCH 1:15 ENVELOPE
* 1 FETCH (ENVELOPE ("Thu, 23 May 2013 07:58:06* 1 FETCH (ENVELOPE ("Thu, 23 May 2013 07:58:06
+0000" "RE: svn commit: r1484852 - in+0000" "RE: svn commit: r1484852 - in
/httpd/httpd/trunk: CHANGES/httpd/httpd/trunk: CHANGES
modules/http/http_filters.c"modules/http/http_filters.c"
* 2 FETCH (ENVELOPE ("Thu, 23 May 2013 10:17:56* 2 FETCH (ENVELOPE ("Thu, 23 May 2013 10:17:56
+0200" {89}......+0200" {89}......
......
* 15 FETCH* 15 FETCH (ENVELOPE ..........(ENVELOPE ..........
a005 OKa005 OK Fetch completed.Fetch completed.

24. Hypertext Transfer ProtocolHypertext Transfer Protocol
HTTPHTTP
➢ 1995 – HTML 2.01995 – HTML 2.0 - RFC 1866- RFC 1866
➢ 1996 – HTTP/1.01996 – HTTP/1.0 - RFC 1945- RFC 1945
➢ 1997 – HTTP/1.11997 – HTTP/1.1 - RFC 2068- RFC 2068
➢ 1999 – HTTP/1.11999 – HTTP/1.1 - RFC 2616- RFC 2616
➢ 2012 – STATUS codes – RFC 65852012 – STATUS codes – RFC 6585
➢ 1995 – URI1995 – URI - RFC 1808- RFC 1808
➢ 2005 – URI2005 – URI - RFC 3986- RFC 3986
➢ Uniform Resource Identifier (URI)Uniform Resource Identifier (URI)
proto :// userinfo @ host : port / pathproto :// userinfo @ host : port / path
user : passuser : pass
useruser

25. Hypertext Transfer ProtocolHypertext Transfer Protocol
HTTPHTTP
➢ Absolute URI or PathAbsolute URI or Path
➢ http://x.com/images/srpr/logo4w.pnghttp://x.com/images/srpr/logo4w.png
➢ /images/srpr/logo4w.png/images/srpr/logo4w.png
➢ Request methodsRequest methods
➢ GETGET
➢ POSTPOST
➢ HEADHEAD
➢ OPTIONSOPTIONS
➢ CONNECTCONNECT

26. Hypertext Transfer ProtocolHypertext Transfer Protocol
HTTPHTTP
➢ Request methodsRequest methods
➢ GETGET
➢ POSTPOST
➢ HEADHEAD
➢ OPTIONSOPTIONS
➢ CONNECTCONNECT

27. HTTPHTTP
➢ Request HeadersRequest Headers
➢ HostHost
➢ Accept-CharsetAccept-Charset
➢ Accept-EncodingAccept-Encoding
➢ AuthorizationAuthorization
➢ RangeRange
➢ RefererReferer
➢ User-AgentUser-Agent

28. HTTP/1.0 vs. HTTP/1.1HTTP/1.0 vs. HTTP/1.1
➢ MethodsMethods
➢ GETGET
➢ HEADHEAD
➢ POSTPOST
➢ MethodsMethods
➢ GETGET
➢ HEADHEAD
➢ POSTPOST
➢ OPTIONSOPTIONS
➢ CONNECTCONNECT
➢ PUTPUT
➢ DELETEDELETE

29. HTTP/1.0 vs. HTTP/1.1HTTP/1.0 vs. HTTP/1.1
➢ HeadersHeaders
➢ AuthorizationAuthorization
➢ RefererReferer
➢ User-AgentUser-Agent
➢ HeadersHeaders
➢ HostHost
➢ Accept-CharsetAccept-Charset
➢ Accept-EncodingAccept-Encoding
➢ AuthorizationAuthorization
➢ RangeRange
➢ RefererReferer
➢ User-AgentUser-Agent

30. HTTP/1.0 vs. HTTP/1.1HTTP/1.0 vs. HTTP/1.1
hackman@terion:~$ telnet google.com 80hackman@terion:~$ telnet google.com 80
Trying 173.194.39.100...Trying 173.194.39.100...
Connected to google.com.Connected to google.com.
Escape character is '^]'.Escape character is '^]'.
GET / HTTP/1.0GET / HTTP/1.0
HTTP/1.0 302 FoundHTTP/1.0 302 Found
Location: http://www.google.bg/Location: http://www.google.bg/
Cache-Control: privateCache-Control: private
Content-Type: text/html; charset=UTF-8Content-Type: text/html; charset=UTF-8
Date: Thu, 06 Jun 2013 08:35:46 GMTDate: Thu, 06 Jun 2013 08:35:46 GMT
Server: gwsServer: gws
Content-Length: 218Content-Length: 218
Connection closed.Connection closed.

31. HTTP/1.0 vs. HTTP/1.1HTTP/1.0 vs. HTTP/1.1
hackman@terion:~$ telnet google.com 80hackman@terion:~$ telnet google.com 80
Trying 173.194.39.99...Trying 173.194.39.99...
Connected to google.com.Connected to google.com.
Escape character is '^]'.Escape character is '^]'.
GET / HTTP/1.1GET / HTTP/1.1
Host: google.comHost: google.com
HTTP/1.1 301 Moved PermanentlyHTTP/1.1 301 Moved Permanently
Location: http://www.google.com/Location: http://www.google.com/
Content-Type: text/html; charset=UTF-8Content-Type: text/html; charset=UTF-8
Date: Thu, 06 Jun 2013 08:36:01 GMTDate: Thu, 06 Jun 2013 08:36:01 GMT
Expires: Sat, 06 Jul 2013 08:36:01 GMTExpires: Sat, 06 Jul 2013 08:36:01 GMT
Cache-Control: public, max-age=2592000Cache-Control: public, max-age=2592000
Server: gwsServer: gws
Content-Length: 219Content-Length: 219
.......Connection still open............Connection still open.....

32. HTTP OPTIONSHTTP OPTIONS
hackman@terion:~$ telnet s1 81hackman@terion:~$ telnet s1 81
Trying 192.168.155.100...Trying 192.168.155.100...
Connected to s1.Connected to s1.
Escape character is '^]'.Escape character is '^]'.
OPTIONS / HTTP/1.0OPTIONS / HTTP/1.0
HTTP/1.1 200 OKHTTP/1.1 200 OK
Date: Thu, 06 Jun 2013 15:13:30 GMTDate: Thu, 06 Jun 2013 15:13:30 GMT
Server: ApacheServer: Apache
Allow: GET,HEAD,POST,OPTIONSAllow: GET,HEAD,POST,OPTIONS
Host-Header: 192fc2e7e50945beb8231a492d6a8024Host-Header: 192fc2e7e50945beb8231a492d6a8024
Content-Length: 0Content-Length: 0
Connection: closeConnection: close
Content-Type: text/htmlContent-Type: text/html

33. HTTP/1.0 requestHTTP/1.0 request
RequestRequest
Response and closeResponse and close
TimeoutTimeout

34. HTTP/1.1 requestHTTP/1.1 request
RequestRequest
ResponseResponse
RequestRequest
ResponseResponse
KeepAliveTimeoutKeepAliveTimeout

35. HTTPHTTP
➢ Request with headerRequest with header
➢ Request with cookieRequest with cookie
➢ Difference between normal and HEADDifference between normal and HEAD
requestsrequests