Every third party program or app needs its own app password to access your information. For the Fastmail app, you need to use your normal password. 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.
- Sending and receiving
- Error on syncing / syncing doesn't work
- Why aren't contacts on my device synced to Fastmail on setup?
- Why don't my fields match up when syncing with CardDAV?
- Password error on syncing
- Manual configuration on iOS