Spark is absolutely killing it with these new features. I switched from using Microsoft Outlook as my primary email client a while ago on both macOS and iOS and it does pretty much everything that I need. Recent updates have included Integrations with a whole bunch of third-party applications and massive improvements to search.
I remain concerned that Spark is free, but Readdle seem to have a game plan where it will earn them an income at some point.
Interesting note, I have been using Postbox for a year but feeling a little fed up with it and looking for an alternative across my iOS devices will have a look at this later. thanks