My Profile Photo

AndrewCz


Using liberty-minded opensource tools, and using them well


Explanation of email servers




A quick overview of all of the moving pieces that make up the internal workings of email servers.

What is an Email Server?

An Email Server is a place that when people send email to you, it gets stored until you can read it. Gmail/Yahoo/MSN all keep your emails there for you to access. This makes it so that anyone can send you a message without you being there to recieve it. It’s called asynchronous communication. But the sent message can’t be floating ‘round the interwebz forever (how we have the internet set up for the time being) so it needs to be stored somewhere. And that somewhere is on the server.

Why not Gmail?

Are you f**king kidding me? In this situation, it’s because you want to have it stored locally. Also, I want the practice in being able to administer the system. But how cool would it be to have your own personal cloud at home. Regardless, for the time being it’s just your own personal email server at home.

Do I need a domain?

Yes. There are two ways to get one it seems though. With a personal internet connection from a consumer-level ISP, the connection will typically be a dynamic one. That means that your IP address will constantly be changing at the ISP’s whim. Either you can hop onto a DDNS server or pay up for a static IP.

Static IP

Having a static IP would be the easiest way to go about doing things. Then again, you’ll need two of them for a functioning email server, and it’ll take some funds to get those IP addresses secured. And after they’re obtained, they would require extensive home network configuration to achieve. This’ll probably be detailed out later here, but for the time being, I will stick with my DDNS setup.

DDNS

Alternatively, you can register with a DDNS server. Typically you can add a subdomain for free, like xxx.no-ip.com or xxx.hopper.pw. Some Examples Replace the “xxx” with your subdomain name, and you can link back to your machine. This is fairly basic…when you’re talking about http pages. It gets much more complicated when it comes to email though.

This will install a client in your server to keep track of which IP address it’s at so that it can forward mail to the appropriate location. Nifty, huh?

Not so fast. As I’ll cover later, it’s really really hard.

What software do I need?

A couple different kinds. I don’t believe in web clients as it teaches the wrong lesson, so I’ll stick to just the basics. This is what I’ll be using:

  • Mutt: to view the message and respond accordingly; the MUA
  • Postfix: to send and receive email; the MTA
  • Dovecot: for IMAP/POP3; the MDA
  • SpamAssassin: to keep spam out of your inbox;
  • ClamAV: to filter out viruses
  • Sieve: to set up mail filters and rules

This is open source though, so there are many other programs that can be used. These are just the ones that I will attempt to make play nice for now.

Mail User Agent

The endpoint of the email process. This is where emails are opened and where emails are sent. The “client”. Examples:

  • Mutt
  • Evolution
  • Kmail
  • Thunderbird
  • Claws-Mail

Mail Transfer Agent

This is the server itself. This is what accepts the email after you hit send, and finds the other MTA that belongs to the recipient’s email address (presumably another MTA). Examples:

  • Postfix
  • sendmail
  • Exim
  • Zimbra

Mail Delivery Agent

Sorting, scanning, filtering, and storing are all under the domain of the MDA. It’s a go-between of sorts between the MTA and the MUA. Examples:

  • Dovecot
  • Cyrus
  • Courier

What are the differences between SMTP, POP3 and IMAP?

SMTP

SMTP is the “Simple Mail Transfer Protocol”. In short, it sends the emails.

POP3 and IMAP

The two of these are the retrieval options that an email client has when it wants to retrieve a message. This one is pretty much black-and-white:

  • POP3 downloads the messages onto the local computer
  • IMAP reads the messages from the remote server

There are advantages and disadvantages to either protocol, and it depends on the use case as to which one would be preferable. For a home mail server, it would be easiest to leave them on the server itself, as it has plenty of disk space.

What about spam and viruses?

  • Spam blacklist server
  • Attatchment scanning
  • Reject unknown sender domain
  • Reject suspicious header messages (cherry-pick-y)

To foreshadow a complication that will be run into down the line, a common rejection test is to match the base domain’s IP address to the IP of which the email originated from. If those two don’t match, the test is not passed, and the mail either gets bounced or dropped.

Going offline

So…what if AT&T on my block goes down for a line repair. Then what happens?

What is an “MX Record”

Do I need MX Records for my sub-domain?

Where is it distributed to?

How does mail get sent through STMP with a DDNS

How does mail get recieved with a DDNS