If it is found, and if the value returned is a literal
RELAY, that domain will be allowed to be relayed:
From:your.domain RELAY ?†? this domain can relay
This feature is fraught with risk. By defining it, you allow anyone on the Internet to
spoof allowed addresses as part of any SMTP MAIL From: command. If you want to
allow local hosts to relay mail from the local network to the world, you can either
authenticate with SMTP AUTH (?§5.1 on page 183) or STARTTLS (?§5.3 on page 202), or
you can relay based on connections using the Connect: keyword in the access database.
7.4.9 Risk with FEATURE(nouucp)
UUCP addresses are those that use a ! character to separate address components.
For example, the following address says to send the message first to the host hostA,
and then hostA will relay that message to the user at hostB:
hostB!user@hostA
If you have tuned your site to prevent unintended relaying, misuse of the nouucp feature
can open your site to an unexpected form of relaying.
Consider a workstation on your network that forwards all its mail to the central mail
hub using LOCAL_RELAY (?§17.5.4 on page 604) or LUSER_RELAY (?§21.9.63 on page 832).
If that workstation also defines:
FEATURE(`nouucp??, `nospecial??)
addresses containing the ! character will not be recognized as special and will be forwarded
to the mail hub as is.
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