Finally Mail for Mac on my computer is working.
What I did was;
- Deleted Fastmail account from Mail app
- Closed Mail app
- Re-opened mail app
- Create a new account (NOT using my Login name, but using the email address I use for all.)
- Account creation failed saying 'Username or Password is incorrect'
- It offered to re-enter username and password, and my email address was also shown.
- Here write the actual username and choose server as ????.fastmail.com
- Leave 'Imap path prefix' empty
- Change SMTP port to 465
- Uncheck 'Automatically detect and Maintain account settings' under IMAP and SMTP 'Advanced' Settings. (Suggested by gj15, Thanks).
- Let the app download mails
All my emails in sub folders are visible now.
I think the new servers and app password ONLY works when you create a new Fastmail account. I had tried changing existing Fastmail account in previous attempts.