Settings - (select each email account in turn) - Fetching Mail - Incoming Server - follow the dialogue with Next as it logs in to the account, and allow permissions. 

I have used Thunderbird (and previous) on Windows Laptop for many years. I have four emails accounts and addresses. I have just installed Thunderbird on android. It had a glitch, but it was unintuitive and not easy to fix. After install using the QR code, the first account was ok but the other 3 had an empty Outbox only. No error messages. The solution turned out to be: Settings - (select each email account in turn) - Fetching Mail - Incoming Server - follow the dialogue with Next as it logs in to the account, and allow permissions. After doing this for email accts 2 3 and 4 it now works. John Gaskell