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Monday, August 12, 2013

IMAP: Downloading emails

Hello friends!

In yesterday's article, "Email: Protocols and Background", I explained how to set up an email client and server to study the email protocols, and also how to use Wireshark to observe how the clients and servers actually talk to each other using these protocols.

Today we'll begin learning IMAP, and use it to access our inbox and download emails.

As I explained in yesterday's article, IMAP is a protocol made up of text commands. You can open a socket (see "C# Network Programming: Simple HTTP Client"), send commands, and interpret the responses. Without needing to write any code, you can use the telnet program to open a raw connection and work using the command line. Connect to your hMailServer IMAP server using a command like this:

telnet serverIP 143

Obviously, replace serverIP with the actual IP address or hostname of the computer where hMailServer is installed. 143 is the port on which the IMAP server listens by default.

If telnet is not installed, you can install it on Windows 7 as follows. In the Start search box, find Programs and Features and open it. Select "Turn Windows features on or off" (it's a link on the left hand side). In the Windows Features window, tick the "Telnet Client" checkbox and click OK.

With telnet installed, connect to the IMAP server as described above. You should see the initial greeting from the hMailServer IMAP server:

* OK IMAPrev1

Now, type the following command and press ENTER:

01 capability

The response consists of a number of words that describe what functionality the IMAP server supports:

Next, login to the email account you created as part of yesterday's article. In my case the command looks like this:

02 login user@ranchtest.local pass

The response will tell you whether the login was successful or not.

In order to make this a little interesting, send a few emails to yourself via Thunderbird so you actually have something in your inbox. Next, access the inbox using the following command. The INBOX folder is standard in IMAP and always exists, even if you don't have anything in it.

03 select INBOX

You should be able to see a few things at this stage. First, whenever we send a command, we precede it with something like "01". This is called a tag, and can be any string (different clients use different formats ranging from numbers to random strings). When the server response to a command, the last line always starts with the same tag as the command - that way a client knows that the response for that command was received.

You'll probably also realise that telnet is a pain in the ass. If you make mistakes you can't backspace - that's because telnet sends everything you type, byte by byte. So each character is sent immediately and can't be undone. Last year I wrote a program called IMAPTalk which makes working with IMAP (and other protocols) much more convenient. Just download it and run it - no installation necessary. Just enter the hostname and connect:

If you turn on Auto-generate tags, you can actually leave out the tags and just type the commands - the tags will be filled in for you by IMAPTalk. If you repeat the commands we did above in IMAPTalk, it looks like this:

Better, no? If you look at the responses (in red), you'll notice there are a bunch of things I haven't explained yet. Don't worry about them just yet. The important thing you should take from the response to the SELECT command is this like:


That's telling us that there are 3 messages in the INBOX.

We can then retrieve them one by one using a FETCH command, while providing the message number:

04 fetch 1 BODY[]

The response is as follows:

You'll notice that there is a bunch of stuff in there, for such a short message. Part of it (the line starting with the * 1 FETCH) is IMAP, as are the last two lines. The stuff in between is the full email, known as the MIME. It consists of a header with a bunch of fields and values (you'll notice important stuff such as From, To, Date, etc), and after a double line break, the message itself. You can see this in Gmail by clicking on the arrow at the top-right of an opened email and clicking "Show original".

You can similarly retrieve the other messages in the inbox by replacing the 1 in the FETCH command with the message number. This is called the message sequence number, and can't be larger than the number of emails in the folder (in this case 3). If you provide a larger number you won't get an error, but you won't get any email data either.

So that's the easiest way to download emails manually using IMAP. We'll learn more about how messages are stored in IMAP folders, and more about the facilities offered by the IMAP protocol, in the coming articles. Come back again for more!

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