Aquamail. Extremely configurable with light, dark, and inky black themes, plus a developer that responds to questions within hours. It's primarily geared towards Gmail but supports Outlook, Yahoo, and others as well and has its own backup option. It's missing the snooze function and doesn't handle Gmail's special categories (Primary, Forums, Promotions) but I don't find this to be a detriment. If you want a client that does support the snooze function and still handle multiple accounts, I suggest BlackBerry Hub. In addition to handling email, you can also add other accounts such as Slack, WhatsApp, Instagram, and Facebook. One of the Hub's perks is it keeps track of individual Slack channels and all, and I do mean all, Instagram notifications. No themes to be found and you're stuck with the font size, but it's very good at what it does.
I tried the default Gmail account but I couldn't make it work without a bulk delete option, which Aquamail and the Hub provide. Inbox is very good, but only supports Gmail accounts. I used to use Inbox alongside Nine, which supports only Outlook/Exchange accounts, but have ditched them both for Aquamail as I prefer having a single app for email. I had previously tried many others like Blue, Typemail, and MyMail but wasn't overly impressed.
Some light reading
regarding email client security, for those interested.