Thank you for this. I actually tried the same thing on my VM last night but had issues connecting to my email account for some reason but didn’t try any more after that. Very happy to see a solid email client working on the phone!
One thing you could try to figure that out is ti run geary on a desktop/laptop computer to verify that the account works there.
If it works on a laptop but not on the phone, then maybe there is some issue with the other GNOME things it is using to handle the account and password info, I was asked to unlock a “keyring”, not sure but probably that’s gnome-keyring being used. Apparently that keyring thing was available and geary was able to communicate with it, in my case. That was just after reflashing the phone and installing all updates. Maybe some very recent updates were needed, that you didn’t have when you tested it?