An "open" SMTP relay is an SMTP server which allows mail to be sent by an offsite
user to an offsite user. Other names for this are "open relay", "open mail server",
"third party relay", "promiscuous mail relay" and so on. This vulnerability is exploited
by spammers (bulk mailers) or anyone who wants to send an untraceable email message.
It is common for spammers to abuse open SMTP relays, sending thousands of untraceable messages
through the server. This can seriously impact the performance of the server, possibly rendering
the server inoperable.
Another consequence of the abuse of this vulnerability is that other organizations, in an
attempt to stop the flow of spam, may throw away any mail originating from your server
(including legitimate mail from your users).