so far I like Mimestream (much better than Apple Mail native); some minor annoyances include the fact that cc and bcc are not turned on standard. Plus the reply button is relatively small (or is that on purpose to ensure users use shortcuts?)
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In the next update, Cc/Bcc have keyboard shortcuts and are "sticky" – once you toggle them on, they stay on until toggled off.
We'll take another look at the toolbar buttons! But the shortcuts are faster ;)