Both are ok in your context.
The two words constitute a noun phrase in which the first noun modifies the second.
It might help you make a choice if you think of the first word as an adjective (although it's not) and the second as a noun.
If there is a preference, therefore, it depends on whether you want to communicate the idea that it is primarily an email (or to emphasize that it's an email, not some other kind of draft), then use draft email. If there's a reason to communicate that it's primarily a draft (or to emphasize that it's a draft rather than some other kind of email), then use email draft.
If you can't think of any reason to make one choice or the other, then use either or use draft email because it's usually thought of as primarily being an email. What kind? A draft email. That's why that expression is somewhat more common.