I always really liked the way Fastcheck worked, but decided to stop using it as it has been almost totally unsupported for years, and I always figured the Fast Services API (on which it was based) would be axed at some stage.
That leaves two current technical solutions for achieving similar functionality:
- Using an email client that supports IMAP IDLE. There are a number of these, but they need configuring to indicate which folders to monitor with IMAP IDLE. Note that relying on a client doing periodic checks of all folders for new messages is not the same thing (and can cause various issues if you have many folders). It is my view, anyway, that using a heavyweight desktop email client just for notifier capability is pretty bizarre.
- Browser notifications. Unfortunately, these are inadequate for power users as Fastmail does not seem to realize people use folders other than the Inbox for receiving incoming emails.
If Fastmail ever finishes (and documents) use of the JMAP backend, it should be possible to write a great notifier using it