From: Chris

>>> the ini_set (or you can set the 5th param to the mail() function) is
>>> return-path. If the message bounces (recipient's mailbox full,
>>> down, whatever the reason) it gets delivered to that address. They
>> serve 
>>> different purposes.
>> The dirty little secret that nobody seems to know is that the RFCs
>> reserve Return-path for use by the MTA servers. It can be replaced by
>> any of them in the routing chain and is used when mail crosses into
>> out of SMTP networks. If you want a return address other than From to
>> stick all the way through, you have to use the Reply-to and/or Sender
>> headers.
> Emails don't bounce to those addresses though do they? I've always 
> thought the return-path is where an email is bounced to and reply-to
> are used by mail clients, not the mta's.

The way I read it, the return-path field is used to indicate a gateway
where the message entered the current email network. So a bounced
message has to be returned to that gateway to get back to the network
from whence it came. This is useful where traffic passes into an SMTP
network from a network with an older routing mechanism, or vice versa.
The intent is to return the bounce to the Sender or From address, in
that order.

But, since the RFC specifically says the field is reserved for use by
the MTAs, it is not proper behavior for an email client to add it to the

Reply-to is a specific instruction to the recipients client software to
insert this address as To when a reply is composed. It is ignored by the

If you don't want bounces to go to the From address, use Sender. It was
intended for mailing lists and administrative assistants to send mail on
behalf of someone else without having to forge their From address. It's
also useful when you have a web site running on a commercial server farm
outside your domain that has to send mail that acts like it came from
inside that domain. Many border filters will block mail when the From
line says it came from within their domain. So the From address can be
the web server (it doesn't even have to be a valid address) while Sender
indicates the address in the domain where any bounces need to be sent.

Bob McConnell

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