Set up a mail server

From Linuxintro
Revision as of 12:16, 11 August 2008 by imported>ThorstenStaerk (→‎verify you can login)

Understand what you are doing

To set up a mail server, you must first be clear about

  • what an MTA is
  • what a MUA is
  • what imap is
  • what pop is
  • what smtp is
  • what an MX record is
  • what sendmail is

To explain all this, here is a little story how a send and a receive of mail could take place: Alice wants to write a mail to Bob. Alice uses kmail as mail program. kmail is her mail user agent, her MUA. She writes to Bob whose address is Of course, the content of her mail is secret so we won't tell it here. After she clicks "send", her MUA transmits the mail to Alice's outgoing mail server. This server has a little program on it called sendmail. This program looks up's MX record. You can do this on your own using the command

dig -t mx

It uses the IP address that it gets and, using the IP protocol, sends Alice's mail there.

Ok, and at this IP address, the message transfer agent of can be reached. This is a server that "speaks" the simple mail transfer protocol SMTP. It can receive mails, and also send them. It has also run on Alice's outgoing mail server. The smtp server receives the mail for Bob and puts it into his mailbox. Bob is asleep at the moment.

The next morning, Bob wakes up and uses his kmail to check his mail. kmail must know how it gets Bob's mailbox. There are two possibilities for that. Either Bob has a pop service running on his server where his mailbox is. In this case, kmail just fetches all mails from the mailbox and (optionally) deletes them. Or Bob has an imap service running there. In this case, Bob gets all mails displayed in his kmail, but they remain on the server. First when Bob presses "delete" in his mail program, the mails are deleted from the mailbox.

Bob can also collect his mails from his various mail accounts around the net. This is where fetchmail comes into the game.

Know your options

  • Well-known MTAs are postfix and sendmail (both use the command sendmail to send mail)
  • Well-known MUAs include kmail and thunderbird
  • imap and pop can be done by courier and cyrus

Set up your mail server

To setup your MTA, install postfix:

yast -i postfix

and run it:

/etc/init.d/postfix start

As a test, send yourself a mail:

sendmail -t root@ << EOF
this is test mail number 1

See if the mail has arrived:

cat /var/spool/mail/root
Content-Description: Undelivered Message
Content-Type: message/rfc822

Received: by (Postfix, from userid 0)
        id 09AAC18BAC06A; Tue,  5 Aug 2008 22:09:03 +0200 (CEST)
Message-Id: <>
Date: Tue,  5 Aug 2008 22:09:03 +0200 (CEST)
From: (root)
To: undisclosed-recipients:;

this is test mail number 1


To see if this has really changed something, we stop postfix now and see if it still works:

/etc/init.d/postfix stop
cat /var/spool/mail/root

It has not arrived. To see that the mail is still in the mail queue, issue the command mailq and see the result:

tweedleburg:~ # mailq
postqueue: warning: Mail system is down -- accessing queue directly
-Queue ID- --Size-- ----Arrival Time---- -Sender/Recipient-------
8BD931648935       88 Mon Aug  4 21:14:48  root

-- 0 Kbytes in 1 Request.

As you see, the mail is still in the mail queue and you get a warning "Mail system is down".

A server is a background program listening on a network port. What was smtp's port again?

tweedleburg:~ # cat /etc/services | grep smtp
smtp             25/tcp    mail         # Simple Mail Transfer

It was 25. Let's see if postfix listens there, first install nmap:

yast -i nmap
nmap localhost

no port 25. Start postfix again:

/etc/init.d/postfix start

And see:

tweedleburg:~ # nmap localhost

Starting Nmap 4.20 ( ) at 2008-08-05 06:19 CEST
Interesting ports on localhost (
Not shown: 1689 closed ports
22/tcp   open  ssh
25/tcp   open  smtp


postfix dies

If you call

/etc/init.d/postfix status 

and get the message "dead", see /var/log/mail.err what happened. If you see something like

postfix/master[5573]: fatal: bind port 25: Address already in use


lsof -i 

to find out what process is blocking the smtp port and kill this process using the command kill.

imap server

set it up

To set up your imap server, use

yast -i courier-imap

configure it



to /etc/courier/imapd. This makes that imap users can authenticate as the local users in /etc/passwd.

start it

/etc/init.d/saslauthd start
/etc/init.d/courier-imap start

verify it is running

tweedleburg:/mnt/loop/suse/x86_64 # nmap localhost

Starting Nmap 4.20 ( ) at 2008-08-05 06:30 CEST
Interesting ports on localhost (
Not shown: 1688 closed ports
22/tcp   open  ssh
25/tcp   open  smtp
111/tcp  open  rpcbind
143/tcp  open  imap

verify you can login

Create a test user to see if you can login to your imap server:

useradd -m testuser

Set password as test password using the command

passwd testuser

Use telnet to connect directly to port 143, the imap-port:

telnet localhost 143
Connected to localhost.
Escape character is '^]'.

Type the imap-command for logging in

1 LOGIN testuser password
* BYE [ALERT] Fatal error: Maildir: No such file or directory
Connection closed by foreign host.

Now do a

mkdir /home/testuser/Maildir
chown testuser:users /home/testuser/Maildir