For more information, from looking at historical emails, it looks like emails with the following subjects are correctly sending both parts
- goodybag purchase confirmation
- Auto top-up confirmation
- Your friend should have received their SIM now
And the following subjects are/were only sending the html part
- We're refunding you for a billing issue
- Check out your monthly usage today
- Top-up confirmation of £10.00
- What's in the blog?
- Did you hear the one about the phones?
- Have you heard about payback?
- Here's an extra GB on us
- Network Update