Sending both HTML and Plain Text versions (MIME) with SendMail command

It's pretty standard if you're sending an email in HTML to also send a plain text version. The SendMail command doesn't seem to be doing this.

While I want HTML viewers to be able to see the styled HTML emails, if a recipient views them as plain text, it just looks like nonsense, which is not really acceptable.

The other option, which is sending text-only emails seems to be the only fool-proof way, but then your business looks budget.

I don't want to use a third party transactional email service when the SendMail command is there.

How can I send multi-part emails direct from Creator? I have tried using MIME markup within the HTML email builder, but that doesn't work.

If it's not possible, can this functionality be added please?