When sending HTML e-mail, I understand that it's a best practice to send a plain text version as well.
But my question is: Must you send a plain text version as well?
What are the repercussions?
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.