The new version also changed the UI for the buttons in the toolbar. Whereas

previous buttons had free-standing defined shapes, the new buttons featured
shapes within a lozenge-shaped capsule. According to many users, and
even Apple's own human interface guidelines at the time, this was worse for
usability. An open-source third-party application that reverted the
icons to their former shapes was available. Nevertheless, Apple updated
their guidelines to include capsule-shaped buttons, and the new UI
persisted.
https://en.wikipedia.org/wiki/Mail_(Apple)
Как в той истории: «Держи, новая спецификация».