It uses ElasticSearch? Does this mean that HEY only supports searching when the device is online? I would expect search to work on a mobile app with no internet access when I'm paying $100 for an email service...
Interesting point. I can't remember the last time I had to search while offline. Not to say that it'd never happen, but implementing a decent index on a mobile device doesn't sound trivial.
I assume this is their web stack. For their mobile apps they most likely use native iOS / Android codebases. The typical stack for a data heavy native app is to use an SQL store (or just filesystem) to cache data locally so its available offline. You won't obviously be able to search all of your emails offline though, only your most recent ones that happened to be loaded during online usage.
why did you assume that the fact that they are using elastic search means they don't support offline search?
You can have both... offline in device when no connection is present and online when it is...