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Mail relaying is the practice of sending mail to a remote mail host other than direct to the recipient host itself or to the host specified in the Domain Name System (DNS) by the MX resource records for the recipient address.

Until recently, most mail hosts on the Internet freely permitted relaying to ensure that the SMTP mail transport system could route around temporary losses of connectivity and other network related problems. However, unscrupulous junk mailers eventually learned to abuse that trust-based system by offloading their messages onto any mail service which offered them the opportunity of hiding the true origin of their messages. It is becoming exceedingly rare, therefore, to find a mail host which offers to accept mail for delivery to anything other than recipients which it regards as being local to itself.

Uncontrolled mail relaying places the viability of your own mail service at risk. If detected and placed on a black list, many mail destinations may refuse to accept mail directly from your host and may also refuse to deliver mail to your host. Further, whilst it is very easy to get on blacklists, getting removed from blacklists can be inordinately difficult, particularly where private schemes are involved. Consequently, understanding the concept of mail relaying is vital for all responsible Internet mail administrators.

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