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Co signed on Aquamail - the original author was very receptive to feature requests and fixing bugs (It was nice when he eventually added scheduled outgoing emails). The software has since been acquired but developement appears to be continuing.

Other clients that caught my eye also were Bluemail or Nine, depending on the need.

I've been using Aquamail on my tablet for years, it's a nice mail app and they continue to support is. I use it on my tablet for the swipe feature especially.

On my smartphone I'm using MailDroid Pro. Like Aquamail, I'm using it for several years now, steady updates and good support. The reason I use MailDroid on my smartphone is the anti-spam plug-in (they charge extra for that though).

I use AquaMail as well, it's one of the few apps I pay for. I paid so I could attach more inboxes though the limit is quite relaxed. The only fancy feature they've added really is a unified mailbox, other than that it's the only android mail client I've used for the past 4-5 years now. I'm a big fan!