In general, since the message that's sent with the null
MAIL FROM generates the RCPT TO from the
inbound MAIL FROM, and there's only one of those, then
you can only get one outbound RCPT TO.
Right.
Wrong. There is no prohibition that I'm aware of against using <> for
other purposes, and there are some standards that specifically require
using <> - e.g. MDNs and responses from mail robots, neither of
which are constrained to exactly one recipient.
FWIW, Keith is correct about this and I found this out the hard way. We had a
customer that absolutely insisted on setting things up to bounce mail with an
empty MAIL FROM and multiple RCPT TOs. There are several ways to do this with
our software and I was experimenting at home trying to find the best one.
Unfortunately, after I finished my experiment and replied to the customer I
forgot to tear all the stuff out of my own configuration. Some time later
I stumbled over it. Of course I removed it but then I got curious and started
looking at my logs. Seems it rejected several messages which, given the
source, pretty much had to be legitimate. Additional followup showed that
at least two of these were valid emails that should have been delivered. The
others were probably MDNs.
Now, there was no reason for these particular valid emails to have had MAIL
FROM:<>, but as Keith says, nothing forbids it either, and it is at present the
only way to say "I don't want any automatic responses to this, thank you very
much".
Ned