Followed the instructions in this article Configure Postfix for DNS Blackhole Lists such as dsbl.org / spamhaus.org database to configure my Postfix email server to stop spam. I used the whole recommendation:

smtpd_recipient_restrictions =
   reject_invalid_hostname,
   reject_non_fqdn_hostname,
   reject_non_fqdn_sender,
   reject_non_fqdn_recipient,
   reject_unknown_sender_domain,
   reject_unknown_recipient_domain,
   reject_unauth_destination,
   permit_mynetworks,
   reject_rbl_client list.dsbl.org,
   reject_rbl_client sbl-xbl.spamhaus.org,
   reject_rbl_client cbl.abuseat.org,
   reject_rbl_client dul.dnsbl.sorbs.net,
   permit

Before that my settings where:

smtpd_recipient_restrictions =
  permit_mynetworks,
  permit_sasl_authenticated,
  reject_unauth_destination

I kept the permit_sasl_authenticated setting too.