Do you have a whitelisting service, SPF (sender policy framework) or some other sending domain validation? SPF is supposed to validate a sending server as authoritative for the “FROM” domain. The SPF record will need to be setup for the sender’s domain.
An example SPF record would look like:
example.com in TXT “v=spf1 a:mailserver mx:example.com ip4:188.8.131.52 include:hostdomain.com ~all”
a:mailserver is the A record name for the authorized server sending mail on behalf of example.com
mx:example.com says that all of the MX records are authorized servers sending on behalf of example.com
include:hostdomain.com means that the ISP for hostdomain.com can also send on example.com’s behalf – BUT hostdomain.com must also have SPF records
~all means that the list above are all of the authorized sending servers for example.com
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