I have used Aquamail https://www.aqua-mail.com/android since the mid 2009 on my android devices. It's been solid and it is super customizable. I think the only downside is they changed their payment model a while back and if you didn't buy it back then it costs monthly fee or a $99.99 for a lifetime license. I highly recomend it if you do a lot of email on your phone.