Which mail clients don't support HTML email directly?
Who cares ? IMHE, HTML email is for Outlook.
Axiom 1: You _must_ give a good non-HTML equivalent to the email.
Axiom 2: Your boss, or whoever is requesting this HTML garbage in the
first place, is clueless enough that they just don;t understand the
problem, or all of the issues involved. If you can demonstrate "HTML
email" once, for once client (Outlook), then as far as they're
concerned, it's working.
For any other non-Outlook, potentially HTML capable email client, then
just let it fall back to the text-only version. After all, you've made
sure that works and is reliable, haven't you.
BTW - Just make sure you don't confuse "readable in Outlook" with
"generated by Outlook". We know how broken Outlook's email is.