The smtpd_relay_restrictions setting applies beforecanonical_maps processing takes place
A reject_non_fqdn_sender restriction in the smtpd_relay_restrictions list will reject unqualified addresses before they can be "cleaned up" by the by the canonical_maps setting.
Worth noting from the Postfix Users mailing list:
smtpd_relay_restrictionssetting applies beforecanonical_mapsprocessing takes placereject_non_fqdn_senderrestriction in thesmtpd_relay_restrictionslist will reject unqualified addresses before they can be "cleaned up" by the by thecanonical_mapssetting.