6.3 Accepting Local Mail from Other Hosts

Most networks have a small number of mail servers that handle the mail for many users who use MUAs on their individual PCs to read and send mail. Outgoing mail from these PCs is sent to the mail server using SMTP, at which point it is the mail server's job to clean up the headers and send the mail on its way.

Locally injected SMTP mail presents two problems. One is to tell which SMTP mail is injected mail from local users rather than the normal incoming mail. This is a crucial distinction, because local users can inject mail addressed anywhere, while incoming mail should be accepted only for the domains that this server handles. (Hosts that promiscuously accept and forward mail from third parties are known as "open relays" and tend to be quickly blacklisted, because the third parties are invariably spammers.) The other, simpler problem is to arrange to clean up the headers in the injected mail the way that qmail-inject or new-inject clean up locally injected mail.