[Spastche]

>but they aren't a reason not to send HTML email

I've seen a lot of email providers flag random emails for having weird HTML. why take the chance of non-delivery at all? send plain text.

[Avamander]

If anything, only plaintext emails are considered suspicious at this point. Some spam filters discourage having mismatching plain text and HTML parts though.

[Fogest]

I've never seen this.