Hi there,   Asking about this for a friend and current customer. Emails he's sending to a specific address are not being received and below is the response he's getting.   Have seen similar issues reported, but have yet to see what the resolution is/was. Think it has something to do with Yahoo email servers blacklisting this email server IP..   (Have removed identifiable details from the contents below) Please let me know if these are required for investigation.     -------- Forwarded Message -------- Subject:Date:From:To: Warning: could not send message for past 8 hours Tue, 26 May 2020 18:01:13 +1000 Mail Delivery Subsystem <MAILER-DAEMON> xxxxxx@tpg.com.au ********************************************** ** THIS IS A WARNING MESSAGE ONLY ** ** YOU DO NOT NEED TO RESEND YOUR MESSAGE ** ********************************************** The original message was received at Tue, 26 May 2020 09:33:59 +1000 from 14-200-244-4.tpgi.com.au [14.200.244.4] This message was generated by mail15.tpgi.com.au ----- Transcript of session follows ----- ... while talking to mx-rogers.mail.am0.yahoodns.net.: MAIL From:<xxxxxxxx@tpg.com.au> SIZE=1401 BODY=7BIT <<< 421 4.7.0 [TSS04] Messages from 203.12.160.61 temporarily deferred due to user complaints - 4.16.55.1; see https://help.yahoo.com/kb/postmaster/SLN3434.html <xxxxxxx@rogers.com>... Deferred: 421 4.7.0 [TSS04] Messages from 203.12.160.61 temporarily deferred due to user complaints - 4.16.55.1; see https://help.yahoo.com/kb/postmaster/SLN3434.html Warning: message still undelivered after 8 hours Will keep trying until message is 5 days old ... View more