What causes this type of address in envelopes?

[Matt Kettler]

Steve Campbell wrote:
> 
> 
> Matt Kettler wrote:
>> Steve Campbell wrote:

> Wow, that's some thick reading.
> 
> So, if this is what they are using, then a simple "reply" to one sent to 
> one of my users should work, unless the sender has their expiry timeout 
> way too short?

This has nothing to do with replies, only error messages like bounces that have 
a null return path (<>).

Replies are sent To: the From: or Reply-To: of the original message and have 
your email address in the Envelope From.

Errors, bounces, DSNs, etc are sent To: the Envelope From of the original 
message, but themselves have a null Envelope From (ie: email address <>).


So, if a message comes in for the "plain" email address, if it's got a non-empty 
return path, it's accepted.

It will only be dropped if it has a null Envelope From, as all such messages 
should be using the tagged address from the envelope, not the plain one in the 
From:.