What causes this type of address in envelopes?

[Steve Campbell]

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?

Thanks in advance.

Steve Campbell