One of the most common problems with outbound mail is configuration of the SMTP service name and DNS records. This post briefly covers the requirements for running a public SMTP service, it does not matter if that server is Exchange, or Sendmail, or PostFix, the same set of rules apply.
The outbound rules do not apply when using a Smart Host to send mail. The inbound rules do not apply if receiving mail through a third-party (such as an anti-spam service).
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