Unwrap SRS e-mail addresses

Unwrap SRS e-mail addresses

When building statistics from mail servers, messages which where
processed by Sender Rewriting Scheme (SRS) are mis-classified as
internal mail while they are external, and appear to come from a huge
number of different addresses which in fact correspond to the same
actual sender.

Add a rewriting function that detects such addresses and extract the
relevant information from it.