I also "love" when I receive "thank you for registering, click here to login", and after staring at the e-mail for a bit, seeing no links anywhere at all, I realize that the well-meaning-but-sadly-still-full-of-fail website operator that sent the message went to the trouble of providing a multipart/alternative with a text/plain body part, but implemented it as "strip the tags from the text/html part: that should be good enough".

I have NO idea of what you're talking about ;-)

Let's pretend you're sending messages to your new users who have just clicked the "Sign Up" button on your fancy landing page.

The problem? They, along with a significant percentage of your other users, use text-based email clients.

This is the wrong way of sending text-only email:

    <!DOCTYPE html> <html> <head> <title>Thanks for signing up!</title> </head> <body><h1>Thanks for signing up!</h1>Click <a href="http://new-registrants.sneakystartup.net/>here</a> to finish your registration. </body> </html>

Stripping HTML tags is also wrong:

    Thanks for signing up!
    Click here to finish your registration.

This is the correct way of sending text-only email:

    Thanks for signing up!
    ======================
    Click here[1] to finish your registration.
    
    1: http://new-registrants.sneakystartup.net

(FWIW, I took the emphasis and subsequent wonkier smily to mean he was joking, and in fact have accidentally sent such emails in the past from his own website.)

Indeed I was.

And most likely you only have a handful of these templates, so if you're nice you'll actually just put some effort into each one:

    Thanks for signing up!
    ======================
    Click here to finish your registration:

    http://new-registrants.sneakystartup.net

(it's much less egregious here than it is in some emails that have 7-8 footnotes)