I am not aware of a spam tool that looks at raw HTML. However, if there is a header that is interesting, you can add that to the filter. For example, Newsgroups is not a default filter option, but since it is a legitimate header entry, I was successful in creating a filter to run off of it. Another approach is software: mailwasher is free for just one account and does an excellent job of spam management. Another option is to create a gmail account, and import your mail through gmail: that lets you then use gmail's filter tools for your account.

I can do this with my own hosted mail server, but means I'll have to import my entire mailbox into a IMAP account and then forward my @hotmail account to it, (I have numerous owned domains).

I have that option! So I'll see if I want to do it.

Thanks for the feedback, but they do need to add this as an option to the filters, and it seems no one else is doing it, so they would be the first, which would be another great reason to use Thunderbird.

Been getting some nasty encoded HTML phishing emails (see attached). No chance of filtering anything due to CSS hidden styles and encoding. I did block the below styles, but apparently legitimate emails do use them, no idea why.

display: none; visibility: hidden; mso-hide: all;

No one should be trying to hide anything in an email, but some companies do, which is annoying!