"Recipient address rejected: User unknown in virtual mailbox table" when sending an email to just one address

RCPT TO failed: : Recipient address rejected: User unknown in virtual mailbox table

This error comes from Postfix attempting a local delivery of the message.

If it happens immediately when trying to send (as opposed to coming back later in a bounce message from a mailer-daemon), it would mean that your SMTP server "thinks" of itself as the final destination for the domain of your client. This is a misconfiguration if your client's mail is not hosted here.

On the other hand, when you send the mail from another account using another SMTP server, the mail is routed to the correct destination based on the DNS MX records of your client's domain.

Maybe the misconfiguration happened by entering your client's domain in Parallels for another purpose and it crept into the mail configuration as a mail domain.

A similar issue is answered here: Mail/Domain config issues - User Unknown in virtual mailbox.

Essentially it's about unchecking a box indicating that that domain's mail is handled by this Parallels instance.