Hmm, so at some point last year, Fastmail bought Pobox. I was a customer of both and already had a Fastmail account at the time.
As part of the acquisition, they created a second new Fastmail account for me with all my Pobox email forwarding aliases carried over.
Now, about 6 months later, I'm finally taking a look at merging that second new account into my existing email account. I managed to move over one of my domains and all my aliases except one.