An other example on modifying messages. I'll show the new header cleanup facilities which are provided. A header contains various kinds of fields: about the contents (body), about the sender and the destinations, etc.
During the transmission process of the messages, groups of fields are
added at each intermediate point: the resent fields. At least,
Received field is added for each set of resent fields.
These fields together form a resent group of fields.
Mailing list will add some fields as well. There are many different mailing list implementations, each adding different fields. These fields can be extracted from the header as list group.