Fastmail is fantastic, no complaints at all.... besides the mobile app. I don't care about the speed or glitches or whatever others may complain about, but I travel way too much to have no offline email access on my phone. So this turns into me having 2 email apps on my phone (Android); one to do stuff in (Fastmail) and another (K-9 which is not great to write/do stuff in) that just sits there, likely hogging battery…
The mobile app is just a native shell around a browser serving up the mobile website -- why can't they have an array of account credentials and allow switching accounts? It's the little things like this that have me annoyed that I'm not just paying cash but also in terms of time and efficiency because of the features Fastmail lacks.
The reason we didn't ship it earlier was around push notifications and the way our accounts are sharded - if you had two different accounts on different backends, merging the pushes into a single badge count and keeping track of the per-device push credentials across the multiple backends was buggy, so it would have provided a sub-standard experience for everybody.