Not a big one, but you can't press "Save Draft" if the Subject field of your message is blank.

Workaround: you can type something in the Subject field, this will activate the Save Draft button. You can then delete the Subject if you want/need to.

(I know it isn't a big problem, just a minor thing. More: I could notice that only because I keep notes and make email drafts to have those notes shared in all of my devices with Zoho Mail app installed, and in the browser).