Email service has a reputation, even among Linux-savvy users and web developers, as being extremely difficult to setup and maintain. The reputation may be well-earned, compared to setting up Apache or MySQL, but email doesn’t need to be something to fear. When you break it down into its component pieces, it becomes far less intimidating, or at least more comprehensible. And, though it seems from the outside that all of the pieces are cohesive, the pieces of a mail server are generally distinct and can be configured without much knowledge of the other pieces.
In this article, I’m going to explain the components of a mail server that can send and receive mail as well as deliver mail to local user mailboxes. Because of the depth of the subject I won’t go into detail about POP/IMAP service for clients like Thunderbird and Outlook, or spam and anti-virus filtering, or DNS for mail service (which is a quite complex subject in and of itself, particularly with DKIM and other email security features that use DNS), but I will cover those in additional articles soon.
MTA, MDA, MUA, IMAP, POP3, SMTP, WTF‽
The first thing to understand about “email service” is that it is not just one service. It is actually made up of several interconnected components, each of which communicates via one or more protocols to other services locally or remotely.
Message Transfer Agent (MTA)
The central component of any email system is the MTA, or Message Transfer Agent, aka Mail Transfer Agent. This is a server that communicates with clients and other servers using SMTP (Simple Mail Transfer Protocol). Some popular examples (in order of popularity and quality of current maintenance at the time of writing) include Postfix, Exim, Sendmail, and QMail. My examples in this article will mostly use Postfix, as it is likely the most popular option, has a good security record, good performance, and is easy to configure.
In its simplest form, the MTA either initiates or accepts an SMTP connection, in order to send or receive an email on behalf of a user. I won’t yet go into the details of the SMTP protocol, though it is a simple protocol when performing simple tasks. For now, it’s enough to know that an MTA like Postfix or Sendmail sends and receives mail via the SMTP protocol. I will show the entire life cycle of an email once I’ve discussed each component.
Mail Delivery Agent (MDA)
Once an email reaches its destination server, the MTA will hand it off to a Mail Delivery Agent, also called a Local Delivery Agent, for delivery to a local user mailbox. The mailbox could be in a variety of formats, Maildir and mbox being the most popular.
There are several mail delivery agents available for Linux and UNIX platforms, with procmail and maildrop being the most popular. The Dovecot and Cyrus mail suites both include a mail delivery agent, in addition to the POP3/IMAP servers they are best known for.
Mail User Agent (MUA)
The Mail User Agent, also simply called “mail client”, is the software used to read (and also send, though via two different protocols) email. Examples of this include Mozilla Thunderbird, Geary, Microsoft Outlook, and Evolution. Other, less obvious examples of mail clients include webmail clients like Roundcube, Squirrelmail, and Usermin, and classic command line clients like mutt and pine. The mail client will use the POP3 or IMAP protocol to retrieve mail from a POP/IMAP server, but will use SMTP to send mail.
There is another class of retrieval software called Mail Retrieval Agent, which includes fetchmail and getmail. This category of tool has become much less popular in recent years, due to evolving user needs, and won’t be covered in this article.
This is another type of MTA which provides a standardized interface for MUAs to retrieve mail from the mail server. Popular examples include Dovecot, Cyrus, and Courier. I will primarily cover Dovecot in this series, as it is among the most popular, very fast and efficient, flexible, easy to configure, and actively maintained. It’s also what we use in a default installation of Virtualmin, so it is what I know best.
A Very Short and Somewhat Boring Story About A Single Email
Once upon a time, the duck wanted to send an email to the fox. So, the duck opened up her email client (Thunderbird, of course), and clicked “Write”. She typed what he needed to say to the fox, and clicked “Send”.
Mail Client to Mail Server Communication
Her mail client then opened a connection to her Postfix mail server running on a server in a data center in Sheboygan, using SMTP. Once established, the conversation between the MUA (Thunderbird) and the MTA (Postfix) went something like this (output from Postfix is indicated with a “S”, as it is operating as the receiving server in this case, while output from Thunderbird is indicated with a “C”, as it is operating as the sending client in this scenario). The server response happens immediately upon opening a connection to port 25 (or possibly other ports in the case of SSL or TLS connections):
S: 220 smtp.fancyduckenterprises.com ESMTP Postfix C: HELO duckhome.wildbirdnetworks.com S: 250 Hello duckhome.wildbirdnetworks.com, I am glad to meet you C: MAIL FROM:<email@example.com> S: 250 Ok C: RCPT TO:<firstname.lastname@example.org> S: 250 Ok C: DATA S: 354 End data with <CR><LF>.<CR><LF> C: From: "Duck" <email@example.com> C: To: "Fox" <firstname.lastname@example.org> C: Date: Wed, 15 Apr 2015 16:02:43 -0500 C: Subject: Fancy Clocks? C: C: Hello Fox, C: Do you have any fancy clocks? C: Your friend, C: Duck C: . S: 250 Ok: queued as 34A1E36F0 C: QUIT S: 221 Bye
This almost looks like a casual conversation between server and client, but there is a reasonably well-defined protocol that determines the negotiation and sending of mail. In the above example, the server and the client both belong to the duck; the next section will cover the conversation between servers that delivers the mail from Duck’s server to a server belonging to Fox.
The first line is the server introducing itself, telling the client the status of the server (220, which means “service ready”), the name of the server (smtp.fancyduckenterprises.com), the protocol that the server speaks (ESMTP is an acronym of Extended Simple Mail Transfer Protocol), and the MTA software (Postfix, in this case).
The next line is simply the client acknowledging the connection by identifying itself with its name (duckhome.wildbirdnetworks.com). “HELO” is simply the protocol defined way this is done.
In the third line, the server acknowledges (250, which means “Requested mail action okay, completed”) the connection and agrees to communicate with the client.
In the fourth line, the client tells the server that it has mail it would like to send, from sender email@example.com.
On the fifth line, the server confirms that it is willing to send mail on behalf of the sender. Postfix (and Sendmail, etc.) has a variety of ways to determine who can send mail and where they can send it from, which I’ll discuss in a later article. Likewise, for the sixth and seventh lines, the client tells the server who the mail is for, and the server agrees to accept it for sending.
The next several lines are the client saying it is ready to send mail data, the server agreeing to accept it (reply code 354, or “Start mail input; end with <CRLF>.<CRLF>”), and the mail data from the client.
Once the client is finished sending mail (indicated by a single “.” on a line by itself), the server queues the mail and gives the client a unique identification number for the mail. This mail ID number can be useful in tracking down mail server troubles. End users rarely need to concern themselves with such implementation details, but system administrators need to know how to troubleshoot mail delivery problems, and mail ID is extremely important for that purpose.
Once queued on the server, the connection is closed, and now the server will attempt to send it to the appropriate mail server for the destination address on the email (the RCPT TO field in the above session).
Server to Server Communication
The Postfix server that queued the mail in the previous section will use the SMTP protocol (again, but as a client rather than a server) to connect to the MTA that serves the foxlocksdocksandclocks.com domain. The sending MTA will use DNS to figure out what server to send the mail to. DNS is out of the scope of this article, but in my next article about DNS I’ll be covering various types of DNS records, including MX records. But, in the meantime, just know that mail servers query DNS for the MX (mail exchange) record of the receiving domain to find out what server to send mail to.
The conversation between the Postfix server on smtp.fancyduckenterprises.com and the MTA (which could be Postfix or any of a wide variety of MTAs) on foxlocksdocksandclocks.com will look strikingly similar to the conversation between Thunderbird and Postfix in the previous example. This is unsurprising, since they both use the SMTP protocol to transmit mail.
So, we won’t go into detail on that part of the story of this particular email, but I will point out that we could find the email in the queue of the server (while it is in the queue, which could be less than a second on a lightly loaded system and if the receiving server is also fast) with the email ID we talked about earlier by using the mailq or postqueue -p command.
Since we’ll be covering POP3/IMAP and client-side communications in the next article in this series, I’m going to skip over that part of the story for now, and get right to the basic configuration needed to achieve SMTP-to-SMTP conversations, both sending and receiving, and we’ll pick up the riveting tale of email ID 34A1E36F0 in the next article.
On modern Linux distributions, installing software is incredibly easy. CentOS has yum, while Debian and Ubuntu have apt. Both are easy to use, automatically resolve dependencies, and will fetch and install software with very little user involvement.
To install Postfix on CentOS, run the following command:
# yum install postfix
Or, on Debian or Ubuntu:
# apt-get update # apt-get install postfix
Assuming your system is connected to the Internet, and the package manager is functional, within a few seconds you’ll have Postfix installed and ready to configure.
A Basic Postfix Configuration
Postfix, freshly installed from OS-provided packages in CentOS, Debian, or Ubuntu (and most other modern Linux distributions) is usually ready to start, and requires little to no configuration. But, since simply saying, “Start Postfix” seems like a cop-out, and this article is for people who want to learn how email works on a deeper level than merely starting the daemon and hoping for the best, I’ll go into more detail about the most commonly changed options in Postfix, as well as some troubleshooting information.
The primary concerns with your initial configuration, the “Just get it working!” phase, is in getting mail accepted for your domain, allowing it to send on behalf of your domain, and delivering mail to the right place on the system, so those are the configuration directives I’ll cover now.
myorigin – This directive determines the origin domain name that is used for mail sent from this system. By default, it will be set to $myhostname, which is a special variable that refers to the hostname of the system (in our example above that would be smtp.fancyduckenterprises.com). But, it may be preferable, particularly if you have multiple sending mail servers, to use the parent domain name (fancyduckenterprises.com), instead. To do that, you would change this option to:
myorigin = $mydomain
Note that this option only effects information that is used during SMTP communication. It does not limit or alter mail being sent through the server; the client sending email determines what the From address will be on any mail sent through your system, and it is that field that determines what appears in mail clients for the recipient of mail. If you only have one mail server for your domain, you probably do not need to change this option, and changing it adds some configuration complexity if you do change it (you have to also setup an alias for each user).
virtual_alias_maps – This directive is used if your mail server will be receiving mail for users in multiple domains. e.g. if you are virtually hosting many mail domains on this server. I’ll go into more detail about virtual domains in a future article, but in the meantime, I wrote a brief tutorial about virtual hosting with Postfix for my Webmin book a few years back, which is still accurate in current Postfix versions. This option will generally point to a local map file, but could also use an LDAP directory or an SQL database on a remote machine, if you have multiple mail servers. In its simplest form, you’d configure it like this:
virtual_alias_maps = hash:/etc/postfix/virtual
Where hash indicates the type of map in use (it could also be ldap, mysql, or one of many other types. The file, /etc/postfix/virtual, will be a specially formatted list of email addresses and the destination for those addresses,
home_mailbox – This directive determines where user mail should be delivered on the system. Most modern systems use the Maildir format for mail delivery, as it can be more efficient for very large mailboxes than the next most popular option, mbox. It also makes incrementally backing up mail systems easier in some cases. If this option has a “/” at the end, it will use Maildir format. Otherwise, it will use mbox. So, to deliver mail to a Maildir in the users home directory, you’d configure it like this:
home_mailbox = Maildir/
That seems simple, and it is for very simple use cases, but in most environments, you will also want to enable spam and antivirus filtering, which generally requires more complexity, possibly using the mailbox_command option, which I will cover in a later article. In the meantime, this one option will allow Postfix to deliver mail to users on the system.
smtpd_recipient_restrictions – The final piece of the puzzle for a basic Postfix configuration is to insure that only the clients you want to allow to send mail can do so. You may have heard of an “open relay”, which is a term for any server that allows non-authenticated users to send mail through the server; being an open relay provides spammers with the ability to send their abusive email through your system (which can have disastrous consequences for your ability to deliver mail). I usually configure this option like so:
smtpd_recipient_restrictions = permit_mynetworks permit_sasl_authenticated reject_unauth_destination
These options determine which networks can send through this system ($mynetworks is another special variable that means the networks that the server is connected directly to, by default, but can be expanded to include other networks), allows SASL authenticated users to send through this system (for clients that are not within $mynetworks), and rejects mail sent to destinations other than the ones this server accepts mail for. Note that these options are applied in order, so the first that matches is the one that applies to any given message. Thus, if the mail is coming from $mynetworks, it will not need to pass the other checks. SASL authentication is a complex subject and will be covered in a future article.
Starting Postfix and Troubleshooting
After configuring the few options that you choose to alter, you can start Postfix and test your installation.
On CentOS 7, Debian 8, and other systemd-equipped systems, you could start it with:
# systemctl start postfix
If there are any problems, you can check the systemd journal using the journalctl command for why. It’s also useful to look in the mail log (/var/log/maillog on CentOS and /var/log/mail.log on Debian/Ubuntu).
On Ubuntu 14.04 and other upstart-based systems, you’d use the start command:
# start postfix
If there are any problems, the mail.log or the system log in /var/log/messages can be consulted to see what went wrong.
A simple way to test a newly installed mail server is to use a local mail client to send mail from the server itself. This provides a very simple test case; external connections and clients are not needed, so you can isolate any failures down to a few specific misconfigurations, rather than trying to guess which part of a very complex system has failed.
There are several ways to send email from the command line, but one that is always available on any Postfix or Sendmail system is the sendmail command (Postfix provides a sendmail program to provide compatibility with Sendmail), so I’ll use that:
$ sendmail -t <<EOF > to:firstname.lastname@example.org > from:email@example.com > subject:Testing > > Tested! > EOF
If things go as expected, you should see the message appear in the inbox of the user you sent the message to. If they don’t go as expected, you can check the maillog or mail.log for clues about why.
This is getting to be quite long, and we still haven’t really dug into client-side protocols, spam and anti-virus filtering, or DNS for mail service. But, fear not, in the next couple of weeks, I’ll be posting two new installments of this series on email, as well as another article about DNS, that cover just that.
Image credit: Kevin Dooley – Mailbox