[log in to unmask] said:

> When there is a permanent mail delivery failure (ie no user, no
> domain, etc) for a user confirmation mail (ie asking them to confirm a
> subscription, for example -- this seems to be the only type), the
> message is bounced to the POSTMASTER of the RELAY (sendmail) machine.

That's because you have sendmail configured to forward double bounces
to the local postmaster.

The SMTP statement MAIL FROM:<> means "don't send me an error message
if you can not deliver this."  Some believe that its use is restricted
to error messages but a careful reading of RFC821 will show that that
is not true.  Its use in sending an error message is given as an
example of where it could be used.  Some have taken that single example
as an exclusionary statement, I don't think it is.

Glenn