The Bat! is an email client for Windows. You can download a trial version from the Ritlabs website.
The Bat! needs its own app password to access your information. If you use your normal password or your Fastmail two step verification password on an external account, syncing to an external service won't work and you will see a password error.
You can follow these help pages from Ritlabs to configure your account for use with Fastmail:
- Set up your mail account using POP or IMAP.
- Sync your contacts
Enter your name, your Fastmail email address, app password, and select if you're using POP or IMAP to fetch your mail.
If you're using a Fastmail domain, The Bat! will automatically fill in all the other settings. If you're using a custom domain, you will need to enter the server and port settings manually.