Fastmail Notifier от iKryten

Fastmail Notifier notifies you of incoming email with (optional) desktop notifications and a badge counter on the toolbar. Selecting the toolbar button opens a preview window allowing you to page through your unread emails where you can choose to delete or mark as read.

Requires a Fastmail account and an API token, which is free to create and takes about a minute in Settings → Privacy & Security → API tokens.

Not affiliated with or endorsed by Fastmail.

Privacy policy: https://github.com/ikryten/fastmail-notifier/blob/master/docs/PRIVACY.md
Bugs can be reported here: https://github.com/ikryten/fastmail-notifier/issues