Help with Postfix Address Verification

I receive "Recipient address rejected: unverified address: mail transport unavailable" when sending to some friends using my new server. I do not receive the error when sending from my old server.

Managed hosting provider has tried various fixes, namely [un]commenting lines in the "smtpd_sender_restrictions" section of main.cf. Thus far, every attempted fix has broken some other functionality of the mail server, and none has prevented the error message (when sending works).

Is there a way to prevent this error message without breaking anything else (ideally while still blocking email for "notauser@example.com")?

Thanks :-)