There appears to be a big problem users are having with passwords right now. I’m also experiencing it. It looks like the stored password is just either lost or changed(?). I have to go into the account multiple times a day and save it again and then restart the app.
I’ve tried fully re-installing and deleting everything with no luck.
v1.10.3
Other posts I found similar:
I poked around a bit. It looks like the password is being set back to an old password. I can only assume this means mailspring is storing / syncing our accounts and the passwords? Where did it even get this password from since I did a full re-install. 
Edit: The old mailspring post says they don’t sync passwords. I’m really confused. Was this a lie?
Logging out of the mailspring id account has solved the problem. So this is obviously a syncing issue.