HTML E-mail: Must you send an alternate plain text view as well?

Ideally you should collect the user preference of email and send the email based on this preference, it is impossible to guess what they prefer. This is not so much a best practice thing as opposed to a user preference, though it is rare now that users have email clients incapable of rendering html content, this does not preclude however email clients blocking html content (like GMail, Outlook) unless explicitly allowed by the user.

You never have to. If the recipient however, is unable to receive HTML emails or their client does not support them then chances are the HTML will be displayed as plain text. Also, some email providers/clients like Verizon, GMail or Outlook will strip out certain tags so you HTML is malformed.

On my humble opinion you must think on the receivers of the mail you are trying to send. I mean, if they have Mail User Agents that support a given format or not. Also, you should be aware of making a compatible HTML e-mail otherwise it won't be interpreted right on every Mail User Agent. Hope that helps,

Think that, for example, Gmail interface shows in the header the first lines of the e-mail, and Outlook does too, if you send an HTML e-mail without plain text version, the text won't be displayed, at least in my case. What I've seen from dozens of e-mails is that they normally include a line that tells: If your e-mail client doesn't support HTML go to http://.... Another thing that I can tell you is that, for example, the iPhone downloads first the plain text message, and later the html formatted one.