Requirements for hosting SMTP servers
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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