🚀 go-pugleaf

In article <1992Nov11.171805.20395@news.acns.nwu.edu>, skrenta@casbah.acns.nwu.edu (Rich Skrenta) writes:
|> I used to think that a!b@c meant "give the message to a with address
|> b@c" (Perhaps that's what smail 2.5 did?).  Lately I've been seeing
|> lots of address binding @ the other way:  "deliver to c with address a!b".
|> 
|> In other words, which is it:
|> 
|> 	a!b@c	->	@c:a!b
|> 	a!b@c   ->	@a:b@c
|> 
|> And what of:
|> 
|> 	b%a@c
|> 
|> Is there a standard which addresses this?  Are most sites giving
|> @ higher precedence, or !?

In RFC 976 - UUCP Mail Interchange Format Standard:

2.1  Hybrid Addresses

   There are (among others) two major kinds of mailing address syntax
   used in the UUCP world.  The a!b!c!user ("bang paths") is used by
   older UUCP software to explicitly route mail to the destination.  The
   user@domain ("domain") syntax is used in conformance to RFC-822.
   Under most circumstances, it is possible to look at a given address
   and determine which sort of address it is.  However, a hybrid address
   with a ! to the left of an @, such as a!b@c, is ambiguous: it could
   be interpreted as (a!b)@c.d or a!(b@c.d).  Both interpretations can
   be useful.  The first interpretation is required by RFC-822, the
   second is a de-facto standard in the UUCP software.

   Because of the confusion surrounding hybrid addresses, we recommend
   that all transport layer software avoid the use of hybrid addresses
   at all times.  A pure bang syntax can be used to disambiguate, being
   written c.d!a!b in the first case above, and a!c.d!b in the second.
   We recommend that all implementations use this "bang domain" syntax
   unless they are sure of what is running on the next machine.

   In conformance with RFC-822 and the AT&T Message Transfer
   Architecture, we recommand that any host that accepts hybrid
   addresses apply the (a!b)@c.d interpretation.