[jonpurdy]

I literally switched on "Enable offline support", caching "All mail" offline on my iPhone a few months ago. Tons of free space, only using 4GB for offline.

But when my phone is actually offline (on a plane or elevator) it beachballs when trying to find something.

[nmjenkins]

(I work for Fastmail). That sounds odd – have you contacted our support team so we can look into it? https://www.fastmail.com/support/

[jonpurdy]

I didn't, but I'll do so now. I usually don't bother with support, because I forget that good companies like Fastmail actually have a competent support team.

[SCdF]

Not OP, but I had to change the mail cache from all mail to recent, I think because it couldn't handle how much mail I had? It would often get stuck not syncing, and offline would be unreliable and showing a blank screen. Recent works fine, but it would be worthwhile folks testing more with all mail enabled, especially if they have larger inboxes.