The forum already sends notification e-mails in plain text, but the mod portal sends HTML mails only. Could we have plain text e-mails there as well? It's less overhead, and I guess most modern MUAs will be able to make links clickable even if they appear in plain text -- even CLI programs like mutt can do that.
Apart from the links, there really seems to be nothing that would justify HTML formatting: there's just no need to use fancy formatting if all that mail should accomplish is to inform people that they've got a message on the mod portal.
A good mod deserves a good changelog. Here's a tutorial (WIP) about Factorio's way too strict changelog syntax!