Hi I am trying to contact postmaster@tpg.com.au because TPG are blocking our emails being sent to all our TPG and all it's associated organisations, but emails to postmaster are being rejected with the following from atmail Cloud Mail System atmail Cloud Mail System   Does anyone know how to contact postmaster ? It looks like TPG are rejecting their own domain.   Thanks Tim   This message was created automatically by mail delivery software. A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed: postmast@tpgtelecom.com.au host au-smtp-inbound-1.mimecast.com [103.96.22.26] SMTP error from remote mail server after RCPT TO:<postmast@tpgtelecom.com.au>: 550 SPF Sender Invalid - envelope rejected - https://community.mimecast.com/docs/DOC-1369#550 [ucNg4mnsOmaDcdVswJExvQ.au24] Reporting-MTA: dns; omr.i-0e5869b43dfedcea0 Action: failed Final-Recipient: rfc822;postmast@tpgtelecom.com.au Status: 5.0.0 Remote-MTA: dns; au-smtp-inbound-1.mimecast.com Diagnostic-Code: smtp; 550 SPF Sender Invalid - envelope rejected - https://community.mimecast.com/docs/DOC-1369#550 [ucNg4mnsOmaDcdVswJExvQ.au24] ... View more