I'm just testing switching from Outlook to Thunderbird, and immediately spotted a design flaw. When looking at a new mail message (I'm on a laptop right now), clicking the delete button causes the toolbar to shift based on the available tools for the next email (essentially, the toolbar is centered). This often causes the "Junk" button to then appear where the "Delete" button was. The reason this is an obvious design flaw is because it then causes the user to have to move their mouse to seek the delete button again. If the user were to be deleting multiple emails, they could very well end up marking some as junk.
This is not my personal opinion. As a 30-year software engineer, I've seen user-acceptance testing pull things like this all the time.
Anyway, only just started testing Thunderbird, but so far so awesome! Outlook has been my go-to since version 1, but in the last few years ALL of Micro$ofts products have become garbage.
Long live the opensource generation.
