Maybe the design needs to adjust with use. Imagine a person new to sending mail has a big and clear send button at the top, then, the UI learns over time how familiar the user is with the interface. If they seem capable of hitting the send button then it can be shrunk down a bit. Rinse and repeat until the button is down to power user size.
Equally, if the user has been away for a month or two, the send button can be made more prominent, to account for the user forgetting the interface.
This could be branded muscle memory, so the send button gets fat unless it is regularly used.