Everything is transferred as text, which means binary files (embedded images, attachments, etc) have to be Base64 encoded. This adds a CPU overhead, but most importantly inflates file sizes by ~30%
Then there's security issues; the vast majority of mail servers default to clear text rather than SSL. But that wouldn't be so bad if there was some inbuilt method for end-to-end encryption. Sadly there isn't, so we have a whole plethora of 3rd party hacks for the mail client - most of which are incompatible with each other.
And while we're on the subject of clients - things are a complete mess there. Trying to design a HTML email that will work on most clients is like trying to design a webpage in the 90s. So we need a more standardised way of displaying that data - and if that means forking HTML for email purposes specifically, then so be it.
Also, horizontal scrolling webpages are counter intuitive and a complete pain for most users who only have vertical scrolling on their mice / trackpads. Quite frankly, if that's an advert of their product design then it leaves me less than excited about the 'innovations' they'd make to email clients.
I know all this seems highly critical against this project - which isn't really the best etiquette on HN. But honestly, why are the old ARPNET protocols (FTP, email, etc) still in widespread use when we have the technology and experience to release vastly improved specifications?