Non-standard headers can leak sender information to anonymous lists.

[Mark Sapiro]

Various MUAs, MTAs and MDAs can add non-standard X- headers to messages, e.g. X-Envelope-Sender, that might reveal sender information in posts to anonymous lists. Since it is not possible to know what all these headers might be, it is not possible to explicitly remove them. Thus, we will instead use a configurable list of headers to keep and remove the rest.