Incoming mail sent from Fastmail rejected, from Gmail accepted

For some reason, when I test my incoming mail-receiver from various Fastmail email addresses, the email doesn’t arrive in my Admin > Emails > Received or Rejected parts and Fastmail sends me an email with an error saying Undelivered Mail Returned to Sender with

    mailuser.nyi.internal[/var/run/lmtpforward/outgoing] said: 550 5.1.1
    <>: Recipient address rejected: User unknown in
    virtual mailbox table via compute4 (in reply to end of DATA command)

But when I send with one of my various Gmail addresses, the email arrives to the site into Admin > Emails > Received.

Any idea why Fastmail might be failing?