[BritTim]

The filter rule works well with one limitation. The forwarding only occurs once the email has entered Fastmail's system. This is not ideal when you want to protect yourself against a catastrophic outage at Fastmail where incoming mail is being queued. For those with their own domain, a better approach is to add the backup account as an additional destination in the domain aliases. When this is done, the MX servers arrange for mail to be delivered to the backup account even when the whole of New York Internet is down.

The same approach is possible for your additional Fastmail aliases (but not your main account login)