Set up a mail server
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 Alice and Bob communicate via mail: 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 firstname.lastname@example.org. 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 home.org's MX record. You can do this on your own using the command
dig -t mx home.org
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 home.org 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.
This little story explains:
- an MTA is responsible for sending and receiving mail
- a MUA is responsible for displaying mail
- for a MUA to be able to display mail, you still need an imap or pop server
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
In this tutorial we use
- postfix as MTA
- courier as imap server
- kmail as MUA
Set up your MTA
We will use the postfix software for the MTA service. We use its default configuration, so every operating system user will get an e-Mail-address in the form username@domain.
yast -i postfix
Should it already be installed, no matter. Now run it:
As a test, send yourself a mail:
sendmail -t email@example.com << EOF this is test mail number 1 EOF
See if the mail has arrived:
cat /var/spool/mail/root [...] --09AAC18BAC06A.1217966943/whatever.dedicated.blah.de Content-Description: Undelivered Message Content-Type: message/rfc822 Received: by whatever.dedicated.blah.de (Postfix, from userid 0) id 09AAC18BAC06A; Tue, 5 Aug 2008 22:09:03 +0200 (CEST) Message-Id: <20080805200903.09AAC18BAC06A@whatever.dedicated.blah.de> Date: Tue, 5 Aug 2008 22:09:03 +0200 (CEST) From: firstname.lastname@example.org (root) To: undisclosed-recipients:; this is test mail number 1 --09AAC18BAC06A.1217966943/whatever.dedicated.blah.de--
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 email@example.com -- 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:
tweedleburg:~ # nmap localhost Starting Nmap 4.20 ( http://insecure.org ) at 2008-08-05 06:19 CEST Interesting ports on localhost (127.0.0.1): Not shown: 1689 closed ports PORT STATE SERVICE 22/tcp open ssh 25/tcp open smtp [...]
Set up Maildirs
We want to use Maildirs as special mailboxes. This is because courier can only handle those. So, change /etc/postfix/main.cf, replace
#home_mailbox = Maildir/
home_mailbox = Maildir/
and restart postfix:
Now look that you can receive mails:
sendmail -t testuser@localhost << EOF This is the 2nd test mail. EOF cat /home/testuser/Maildir/new/* [...] This is the 2nd test mail.
Now you can use kmail to read your mails.
Let's assume you want to receive mails for firstname.lastname@example.org. As long as postfix does not know it is supposed to accept mails for the destination venus.org it will not do it. To tell postfix to accept mails for venus.org, set the variable mydestination in /etc/postfix/main.cf like this:
mydestination = $myhostname, localhost.$mydomain, venus.org
Maybe you want webmaster@yourdomain and info@yourdomain to be delivered to yourname@yourdomain. In this case, install webmin and choose Servers -> Postfix -> Mail aliases like shown below:
If you call
and get the message "dead", see /var/log/mail.err what happened. If you see something like
postfix/master: fatal: bind 127.0.0.1 port 25: Address already in use
to find out what process is blocking the smtp port and kill this process using the command kill.
Set your MX record
If you want to receive mail that goes to the address email@example.com, you need to set the MX record of domain.org. The MX record must be the ip address of the server where the MTA is running. To find out your current MX record, use dig:
tweedleburg:~ # dig -t mx linuxintro.org ; <<>> DiG 9.4.2-P1 <<>> -t mx linuxintro.org ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 9367 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1 ;; QUESTION SECTION: ;linuxintro.org. IN MX ;; ANSWER SECTION: linuxintro.org. 86400 IN MX 50 mx0.linuxintro.org. ;; ADDITIONAL SECTION: mx0.linuxintro.org. 86400 IN A 22.214.171.124 ;; Query time: 59 msec ;; SERVER: 126.96.36.199#53(188.8.131.52) ;; WHEN: Sat Sep 20 14:43:23 2008 ;; MSG SIZE rcvd: 68
In this case, the MX record points to mx0.linuxintro.org whose ip address is 184.108.40.206
Before you set the MX record, all mails arriving were addressed to whatever@localhost. Now, say we are talking about mydomain.org, mails arriving can also be addressed to mydomain.org. We need to tell postfix to accept those. Modify /etc/postfix/main.cf:
mydestination = $myhostname, localhost.$mydomain, mydomain.org
and restart postfix:
Next thing is that you tell your server to accept connections from the public internet. Modify /etc/postfix/main.cf:
inet_interfaces = localhost, mydomain.org
and restart postfix:
verify the network is open to the outside:
netstat -putan | grep 25 tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN 18022/master tcp 0 0 220.127.116.11:25 0.0.0.0:* LISTEN 18022/master
In this case, 18.104.22.168 will be replaced by your IP address.
Using Spamhaus/Greylisting for blocking
Adding Spamhaus to smtpd_recipient_restrictions:
smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination, reject_rbl_client zen.spamhaus.org, <- Spamhaus Blocklist check_policy_service inet:127.0.0.1:60000 <- Greylisting
Now strange users (i.e. users which cannot be identified with SASL) will be blocked, if they are listed in the Spamhaus Directory and are subject to Greylisting.
What comes next?
- set up an imap service - so you can finally read your mail
- set up your mail server for sending - so authenticated users can send mail
- set up webmail - so you can write and read mail from any internet cafe (using a browser)
- set up a web calendar - you want a calendar to accompany your webmail
- fight spam - now it's time to fight spam
- collect mails from other accounts