Rainloop is an okay, simpler webmail client. It is lacking in a lot of features that Roundcube has, but sometimes you just want easy access to your email when you're away from your full-featured desktop client.

K-9 is good; it's going to be rebranded as Thunderbird for Android soon. FairEmail is also good. It has a different set of trade-offs from K-9, so better for some people, worse for others.