Isn't the idea behind blocking images not to let spammers know that your e-mail address is valid, upon your opening the message and sending an identifiable download request?

I would have assumed a 'block images' setting would block all external references -- to stylesheets, fonts, anything.

Do @import and <link> actually work even if you have images blocked?

Not a big surprise that Apple Mail, iOS Mail, and Thunderbird are the winners here. They're the clients most likely to allow HTML through verbatim in most cases.

Sadly, at least in my case, most people are using Gmail on the Web and they filter the HTML in numerous mystifying ways (although this is ultimately good for us as users IMHO).

Luckily there's still a lot that most mass e-mailers can do, including myself, to make e-mail pleasant for readers without niceties like custom fonts.. but one day it would be great to universally expect something a little more elaborate than HTML 3.2 ;-)

Please no, just stop. HTML in email is an awful idea. Get PDF/A attachments standardised in the client for presentation purposes, if you really must, but No Scripting.

Any idea on stuffing a web font into a multipart message so the tracking-adverse email clients can still use it? Could be impracticable depending on volume anyway, 18k (size of font in example) extra per message isn't nothing.

html email should be banned