Mimecast / SMTP error code 550
smtp;550 DMARC Sender Invalid - envelope rejected - https://community.mimecast.com/docs/DOC-1369#550 [#]
High severity
The inbound message has been rejected because the sending IP address isn't listed in the published SPF records for the sending domain (Return-Path header domain). Ensure all the IP address for your mail servers are listed in your SPF records.
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