Postfix is a free and open-source mail transfer agent (MTA) that is commonly used on Linux systems. It handles receiving and delivering email by using several server processes and queues. When receiving mail, Postfix uses smtpd, qmqpd, pickup, and cleanup servers to validate messages and add them to the incoming queue. For delivery, it uses qmgr to route messages from the incoming queue through active delivery agents like smtp, lmtp, local, and virtual to recipients or deferred queue if delivery fails. Postfix prioritizes stability, scalability and security in its flexible and modular design.

1. Postfix on Linux

2. What is Postfix?

3. Introduction
• E-mail is easily the most popular and
important Internet service today, which
has made it a popular target of cyber-
criminals and spam-happy miscreants.
• It facilitates us to communicate easily
and effectively.

4. Introduction (cont.)
• Internet e-mail is built from several standards and
protocols that define how messages are composed
and transferred from a sender to a recipient. There
are many different pieces of software involved, each
one handling a different step in message delivery.
Postfix handles only a portion of the whole process.
Most e-mail users are only familiar with the software
they use for reading composing messages, known as
a Mail User Agent (MUA). Examples of some common
MUAs include Pine, Netscape Communicator and
Outlook Express. MUAs are good for reading and
composing e-mail messages, but they don't do much
for mail delivery. That's what Postfix fits in.

5. What is a Mail Transfer Agent -MTA?
• MTAs move e-mail from one host or network
to another. These are in contrast to Mail
Delivery Agents, which move mail within a
system (i.e., from an MTA to a local user's
mailbox, or from a mailbox to a file or
directory). In other words, MTAs are like the
mail trucks (and airplanes, trains, etc.) that
move mail between post offices; Mail
Delivery Agents are like the letter-carriers who
distribute the mail to their destination mail
boxes.

6. What is a Mail Transfer Agent (cont.)
• In addition to MTAs and MDAs, there are also
various kinds of e-mail readers, including POP,
POP3, and IMAP clients for retrieving e-mail
from remote systems. These are also known as
Mail User Agents, or MUAs. (There is no real-
life simile for these, unless your mail is handed
to you each day by a minion whose sole duty
is to check your mail box now and then!) But
we're not concerned with these or with
MDAs, except to mention how they relate to
MTAs.

7. What is Postfix?
• Postfix is a free open source mail transfer agent
(MTA), a computer program for the routing and
delivery of email.
• Postfix is the default MTA for a number of Unix(-
like) operating systems such as Ubuntu Linux.
• This mail transfer agent postfix, provides an
alternative to send mail that is simpler in design,
more modular, easier to configure and less work
to administer. Equally important, it's been
designed with scalability, reliability and sound
security as fundamental requirements.

8. What is Postfix? (cont.)
• Postfix is a Mail Transfer Agent and it is the default
Mail Transfer Agent (MTA) for Ubuntu. It is in Ubuntu's
main repository, which means that it receives security
updates. This is supporting LDAP, SMTP AUTH (SASL),
and TLS and running in a chroot environment. Postfix
is Wietse Venema's mailer that started life as an
alternative to the widely-used Send mail program.
Postfix attempts to be fast, easy to administer, and
secure, while at the same time being send mail
compatible enough to not upset existing users. Thus,
the outside has a sendmail-ish flavor, but the inside is
completely different. This software was formerly
known as VMailer. It was released by the end of 1998
as the IBM Secure Mailer. From then on it has lived on
as Postfix. This rpm supports LDAP, SMTP AUTH (trough
cyrus-sasl) and TLS. If you need MySQL too, rebuild
the srpm --with mysql.

9. Founder of Postfix
• POSTFIX was originally written by Wietse
Venema during a stay at the IBM Thomas J.
Watson Research Center. Postfix was first
released in mid-1999.
• Dr. Wietse Zweitze Venema (born 1951) is a
Dutch programmer and physicist best known
for writing the Postfix mail system. He has also
written numerous other security related tools,
such as SATAN and The Coroner's Toolkit,
both in co-operation with Dan Farmer, and
TCP Wrapper.

10. Dr. Wietse Zweitze Venema

11. • He studied physics at the University of
Groningen, graduating with a PhD. He
spent 12 years at Eindhoven University
as a systems architect in the
Mathematics and Computer Science
department, and spent part of this time
`
writing tools for Electronic Data
Interchange. Since emigrating to the
U.S. in 1996 he has been working for the
IBM Thomas J. Watson Research Center
in New York.

12. He was awarded in many awards such
as;
• Security Summit Hall of Fame Award
(July 1998)
• SAGE Outstanding Achievement Award
(November 1999)
• NLUUG Award (November 2000)
• Sendmail Milter Innovation Award
(November 2006)

13. Developed Wietse Venema and many
by others
Latest release 2.5.3 / July 29, 2008
Preview
2.6-20080726 / July 26, 2008
release
OS Cross-platform
Type Mail transfer agent
License IBM Public License
Website http://www.postfix.org/

14. Structure
•How Postfix receives mail
•How Postfix delivers mail
From Postfix page

15. Receives
Delivers

16. How Postfix receives mail
trivial-
rewrite(8)
Network -> smtpd(8) ^ |
| v
Network -> qmqpd(8) -> cleanup(8) -> incoming
/
pickup(8) <- maildrop
^
|
Local -> sendmail(1) -> postdrop(1)
From Postfix page

17. How Postfix receives mail
Network -> smtpd(8)
cleanup(8)
Network -> qmqpd(8) ->
smtpd(8) or qmqpd(8) servers :-
• enter Network mail to postfix.
• remove the SMTP or QMQP protocol encapsulation.
• enforce some sanity checks to protect Postfix.
• block unwanted mail.
• give the sender, recipients and message content to
the cleanup(8) server.
From Postfix page

18. How Postfix receives mail
cleanup(8)
/
pickup(8) <- maildrop
^
|
Local -> sendmail(1) -> postdrop(1)
sendmail(1) compatibility command :-
• receive Local submissions.
From Postfix page

19. maildrop queue :-
• queued Local submissions by the privileged
postdrop(1) command.
pickup(8) server :-
• picks up local submissions.
• enforces some sanity checks to protect Postfix.
• gives the sender, recipients and message content
to the cleanup(8) server.
From Postfix page

20. trivial-
rewrite(8)
^ |
| v
cleanup(8) -> incoming
cleanup(8) server :-
• Accept mails from smtpd(8), qmqpd(8) and pickup(8).
• Accept Mail from internal sources.
• implements the final processing stage before mail is queued.
trivial-rewrite(8) server :-
• rewrites addresses to the standard "user@fully.qualified.domain"
form.
From Postfix page

21. How Postfix delivers mail
trivial- smtp(8) -> Network
rewrite(8) /
- lmtp(8) -> Network
^ |
| v /
incoming -> active -> qmgr(8) - local(8) -> File, command
^ |
| v - virtual(8) -> File
deferred
pipe(8) -> Command
From Postfix page

22. qmgr(8) (queue manager ) :-
• This is the heart of Postfix mail delivery.
• It contacts the smtp(8), lmtp(8), local(8), virtual(8), pipe(8),
discard(8) or error(8) delivery agents.
• sends a delivery request for one or more recipient addresses.
discard(8) and error(8) delivery agents :-
• discard or bounce all mail.
active queue :-
• maintains by the queue manager with the messages that it has
opened for delivery.
• acts as a limited window on potentially large incoming or deferred
queues.
• prevents the queue manager from running out of memory under
heavy load.

23. deferred queue :-
• maintains by queue manager for mail that cannot be delivered.
trivial-rewrite(8) server :-
• resolves each recipient address according to its local or remote
address class.
• recipients whose address has changed; mail for such recipients is
returned to the sender with an explanation.

24. Behind the scene
These server processes rely on other
server processes that do things behind
the scenes.
As before, names followed by a number
are Postfix commands or server
programs, while unnumbered names
inside shaded areas represent Postfix
queues.

25. – The resident master server is the supervisor that
keeps an eye on the well-being of the Postfix mail
system.
– The bounce, defer and trace services each
maintain their own queue directory trees with per-
message log files.
– The proxy map servers provide read-only and
read-write table lookup service to Postfix
processes.
– The scache server maintains the connection
cache for the Postfix smtp cleint.

26. When delivering mail to a destination
with multiple mail servers, connection
caching can help to skip over a non-
responding server, and thus
dramatically speed up delivery.

27. Postfix Queue
• A key contributor to the stability and
the speed of postfix is the intelligent
way in which it queues mail.
• Postfix uses four different queues,
» Mail drop queue.
» Incoming queue.
» Active queue.
» Deferred queue.
27

28. Mail drop queue
• Mail that is delivered locally on the system is
accepted in the Mail drop queue.
• In this queue, the mail is checked for proper
formatting and fixed if necessary.
• Then it is handed to the Incoming queue.
Mail Drop queue Incoming queue
Mail
28

29. Incoming queue
•The Incoming queue receives mail from
other hosts, clients or the Mail drop
queue.
• As long as e-mail is still arriving and as
long as postfix hasn't really handled the
e-mail, this queue is the place where
the e-mails are kept.
29

30. Active queue
•The Active queue is the queue that is
used to actually deliver messages and
therefore has the greatest potential risk
of something going wrong.
•This queue has a limited size, and
messages will be accepted only if
there is space for them.
•That means e-mail in the Incoming and
deferred queues have to wait until the
Active queue can accept them.
30

31. Deferred queue
•E-mail that cannot be delivered is
placed in the deferred queue.
•This prevents the system from
continuously trying to deliver e-mail
and keeps the Active queue as short
as possible in order to give newer
messages priority.
•This also enhances stability of the mail.
31

32. Deferred queue continued…
•If the MTA cannot reach a domain, all
the e-mail for that domain is placed in
the deferred queue, so that those
messages will not needlessly
monopolize system resources.
•Retry is scheduled with an increasing
waiting time. When the waiting time
expires, the e-mail is again placed in
the Active queue for delivery.
•The system keeps track of retry history. 32

33. Mail drop queue Incoming queue Active queue
Mail
Deferred queue
33

34. Postfix Commands
•The most common invocations of the
postfix command are postfix start,
postfix stop and postfix reload.
•Postfix Start- This command will start the postfix. If
you already not installed postfix, terminal will
display an error.
•Postfix Stop- This command will stop postfix
through the action.
•Postfix Reload- This will reload its configuration
files without stopping and restarting.
34

35. • subhash@subhash-laptop:/root$ postfix start
subhash@subhash-
The program 'postfix' is currently not installed
• You can install it by typing:
– sudo apt-get install postfix
apt-
35

36. Postfix Commands
continued…
•There are also some other commands,
•Postfix flush- This will immediately attempt to
send all queued messages.
•Postalias- Maintains Postfix alias databases.
•Postcat- Displays the contents of Postfix queue
files.
•Postkick- Makes some internal communication
channels available for use in.
•Postlock- Provides Postfix-compatible mailbox
locking for use in.
36

37. Features of Postfix
Security
Simplicity
Compatability
Robustness
Stability

38. Postfix Content Inspection
Postfix supports three content inspection
methods
1. Before queue, built-in, light-weight
2. After queue, external, heavy-weight
3. Before queue, external, medium-
weight

39. Certain Limitations
Content Inspection Software must finish
in a limited amount of time
Content Inspection Software must run
in a limited amount of memory
Befor queue inspection limits the
sophistication of the content filter that
you can use

40. Comparison of mail servers
SMTP POP APO File Data LDA Oth
P syst bas P er
em e
MS Yes ? Yes No No Yes Active
Directo
Ex: ry
Serv
eDirect
Net Yes Yes Yes Yes Yes Yes ory,
Mail LDAP
Post Yes No No Yes Yes Yes ?
fix

41. Yes Yes Yes Yes
Surg ? ? ?
e
Mail
Yes Yes Yes Yes Yes No NT
Win Domain
Gate , Active
Director
y,
CRAM-
MD5,
SASL
PLAIN,
SASL
LOGIN

42. Feature comparison
Lin Wi M SM PO IM SM P o NN SSL W
ux nd ac TP P3 AP : : TP eb
ow OS TLS TLS m
s X ail
MS No Yes No Yes Yes Yes Yes Yes Yes Yes Yes Pro
Ex: Ser prie
tary
Net Mail Yes Yes No Yes Yes Yes Yes Yes Yes Yes Yes Pro
prie
tary
Yes No Yes Yes No No Yes No No Yes No Op
Postfix en
sour
ce

43. Publi
g Yes No Yes Yes Yes No No No
? No No
c
m dom
ail ain
Ope
Se Yes No Yes Yes No No Yes No No No No
n
nd sour
ce/IB
m M
Publi
ail
c
Lice
nse
Propr
Sp Yes Yes Yes Yes No No Yes No No Yes No
ietar
ar y
k
En
gi
ne

44. Conclusion
• is a Mail Transfer Agent
• use to route email
• simple to configure
• Highly respect by expert for its secure design & facts
• Use to troubleshooting

45. Thank you