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Your mail server’s forward and reverse DNS name must be identical.
It should be a real host name, not a generic and dynamic name given to you by your provider, like ip-123-123-123-123.myprovider.com.
It may use the same top level domain name as your Email sender address but this is not mandatory.
Your domain name must have an existing and working MX record. Though it does not have to be the same server as the one you use to send your Emails.
That The mail server specified in the MX record must have a working “postmaster” address.
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