What causes this type of address in envelopes?

[Matt Kettler]

Steve Campbell wrote:
> I have a little problem with a from envelop address. All of the header 
> addresses are normal type somebody at somedomain.com, but the envelop from 
> address takes the form
> 
> prvs=somebody=7848db996 at somedomain.com.
> 
> The recipient is a little mad, because I had the address whitelisted 
> using the normal type address and it used to work. They want me to 
> whitelist it again. I guess I could resave my file, but....
> 
> Is it a proxy, or one of those dang MS products? Any clues?

It's the Simple Private Signature (prvs) part of a IETF draft Bounced Address 
Tag Validation (BATV).

Essentially, it's a way of dealing with fake bounce messages by always using a 
mangled address when sending. Any bounce messages that come back which are sent 
to the regular user address are obviously caused by spam forgeries (or broken 
mailsystems) and can be dropped.

The current draft spec can be found at:
http://tools.ietf.org/html/draft-levine-mass-batv-02