Message text rendering poorly
The text for messages' subjects and previews seems to be image-based, instead of actual text. (Not vector-rendered) It therefore looks unpleasant and blurry-edged at high resolution. Possible solutions could be reverting to displaying message subjects/previews in plain text. (or some other vector-rendered solution)
D C commented
I second this. The new mail text is very fuzzy and hard to read.