Is it possible to load and read messages while offline?
You can load emails in advance by clicking specific folders; the 25 most recent conversations in each folder you click will remain loaded while offline. It’s also possible to compose replies offline. However, if you close the app, reopening it while offline won’t show any emails. But your offline draft edits won’t be lost. When you get an Internet connection again, emails will load, and your drafts will be available again.
This can be somewhat workable on a computer but is trickier on mobile since the device can decide to close the app whenever it’s put in the background (ie. you move to another app or the device goes to sleep). So if keeping your emails loaded is critical at a given moment, try to keep Missive always in the foreground and prevent the device from going to sleep.
You can upvote the feature request for better offline support here.