I like to compose and read email with a fixed width font. Gmail now supports reading a message with a fixed width font using the Show in fixed width font option - but you have to do that on each message as you read it.

If you use Firefox, you can create a user stylesheet that is domain specific to force all non-HTML messages and the message composition textarea to display in fixed width.

The user stylesheet should contain:

@-moz-document domain(mail.google.com) {
    /* GMail messages and textarea should use fixed-width font */
    textarea.dV, div.ii.gt {
        font-family: MonoSpace !important;
        font-size: 9pt !important;
    }
}

Place the stylesheet (on MacOS) under your Firefox profile directory e.g. ~/Library/Application Support/Firefox/Profiles/XXXXXXXX.default/chrome/userContent.css