My history with email clients being acquired is not encouraging. The history is they are effectively abandoned and/or shut down on the order of weeks and months not years. See Dropbox/Mailbox.
Ignoring the constraint of "email client", has there been any acquisitions where the acquired product got better post-acquisition?
I can think of countless examples where it got worse, but from the top of my head, I can only think of maybe YouTube, but then only in the initial post-purchase period, and same goes for a bunch of other examples. They seem to eventually always turn sour.
Maybe GitHub? But it traded "no new features - no downtime" for "some new features - a lot of downtime" after the Microsoft purchase, so I guess it's very subjective, probably at least some people like that tradeoff.
Diamond Aircraft
Volvo
Cirrus
All retained their culture and brand and the products keep improving incrementally. Parent companies keep a low profile wrt product.
And Google/Sparrow.