Gmail – How to Change Default View to Plain Text

Update 2019-07-15:

The gmail-fixed-font extension is now hosted on github: https://github.com/jparise/gmail-fixed-font

The instructions are provided in the repo README, but to summarise (assuming you are using Chrome):

1. Download the script
2. Open a new tab and navigate to chrome://extensions/
3. Enable Developer Mode
4. Drag and drop the script onto the window and install
5. (optional) you may want to allow the extension only for the gmail domain by clicking Details > Site Access: On specific sites and enter https://mail.google.com in the dialogue box and save

Ok, you have to try Gmail Fixed Font userscript

// ==UserScript==
// @name           Gmail Fixed Font
// @namespace      http://www.indelible.org/
// @description    Fixed-font message bodies for Gmail
// @author         Jon Parise, James Tunnicliffe
// @version        1.3
// @include        http://mail.google.com/*
// @include        https://mail.google.com/*
// @include        http://*.mail.google.com/*
// @include        https://*.mail.google.com/*
// @grant          GM_addStyle
// ==/UserScript==

// Plain-text Message Body
var css = ".ii, .Ak { font: medium monospace !important; }";
// Compose Interface
css += ".editable { font: medium monospace !important; }";

if (typeof GM_addStyle != "undefined") {
    GM_addStyle(css);
} else if (typeof addStyle != "undefined") {
    addStyle(css);
} else {
    var heads = document.getElementsByTagName("head");
    if (heads.length > 0) {
        var node = document.createElement("style");
        node.type = "text/css";
        node.appendChild(document.createTextNode(css));
        heads[0].appendChild(node);
    }
}

In Gmail, I think your only option is to click the arrow on the top right of the message, and then choose "Show Original".

The message is most likely to be sent in MIME format, so you can scroll down past the headers and look for something like this: Content-Type: text/plain. In MIME format, there are unique strings (boundaries) between each message part, I believe that each email client chooses its own string to use. You can find out what is being used if you locate the following header:

Content-Type: multipart/alternative;
        boundary="-----------=Sample_Msg_Part156165161321654"

In this case, the string -----------=Sample_Msg_Part156165161321654 is used to delimit the different message parts.

Here's an example...

Let's say that the message has the following content:

From: user@example.com
To: user@example.com
Subject: Test
MIME-Version: 1.0
Content-Type: multipart/alternative;
        boundary="-----------=Sample_Msg_Part156165161321654"

-----------=Sample_Msg_Part156165161321654
Content-Type: text/plain

This is a sample message. This is the text portion of 
the message.
-----------=Sample_Msg_Part156165161321654
Content-Type: text/html

This is a sample message. This is the <b>html</b> portion
of the message.
-----------=Sample_Msg_Part156165161321654

... the plain text would look like this:

This is a sample message. This is the text portion of the message.

... while the HTML would look like this:

This is a sample message. This is the html portion of the message.