How do I send email in plain text rather than HTML in my email client?
Thunderbird (per-message)
Shift-clicking the Write, Reply or Reply All button (but, oddly, not the Forward button) opens the message editor window using the opposite of the current setting, i.e. HTML if set to plain text, and vice versa.
(It would be so much easier if they had a drop-down box, rather than this convoluted method).
Gmail:
Very easy. Persists between sessions.
Thunderbird
Go to Tools -> Account Settings, and under your account choose Composition and Addressing. Uncheck Compose messages in HTML format.
Repeat for all accounts you have, then click OK.
Oh, and I also have an ASCII Ribbon Campaign link in my .signature too.
For Outlook, you can set to have all new emails be written by default in plain text format, or just that one email.
To have all new emails be sent in plain text format: go to Tools > Options > Mail Format tab, and check the option that says "Create all new email messages in plain text format".
To write a new email message or reply to one using the plain text format:
If you use MS Word as your editor when writing emails (you will see this if you have the buttons for Bold/Italic/Underlined text in a bar above the message field), look for the drop down that has HTML/Rich Text/Plain Text as choices. Select "Plain Text," and if you get a message saying choosing it will make the email lose all its current formatting, click Yes.
If you're not using MS Word as your editor, click on Format, where you can choose between Plain Text and either Rich Text or HTML (this is usually determined by what is set by default - see my answer above).
My answer is based on my experiences with Outlook 2000, XP, and 2003. I have not used anything from Office 2007, but I'm guessing you should have the same options.
Within Gmail it defaults to plain text. To get HTML formatting click on "Rich formatting" just above the email body. If it is already in "Rich formatting" mode there will be a "Plain Text" link above the email body. Clicking that will change it back to plain text mode.