I am having the same/similar problem sending to Bigpond accounts from my tpg account using outlook 2016. If I send from my Gmail account using outlook 2016 I don't have a problem.   The original message was received at Wed, 2 Feb 2022 10:16:21 +1100 from 118-208-192-227.tpgi.com.au [118.208.192.227]   This message was generated by rpt-glb-mail01.tpgi.com.au      ----- The following addresses had permanent fatal errors ----- <xxxxxxxxx@bigpond.com>     (reason: 550 5.7.1 Connection refused. 60.241.0.26 is blacklisted in ReturnPath RPRBL List. See https://senderscore.org/blocklistlookup/)      ----- Transcript of session follows ----- ... while talking to extmail.bigpond.com.: <<< 550 5.7.1 Connection refused. 60.241.0.26 is blacklisted in ReturnPath RPRBL List. See https://senderscore.org/blocklistlookup/ 554 5.0.0 Service unavailable ... View more