At the risk of starting another "should messages survive mailing lists"
thread, which I don't think would be productive, let me just point out:
Robert Barclay wrote:
If there are cases where a message may be altered by someone other than
those people over whom they can exercise control (either their employees or
people with whom they have a direct relationship) then to get them to adopt
mailsig the list of those transformations will need to be pretty limited and
it will be necessary to be able to enumerate them to the non-technical
managers of those companies in a way that makes it clear that adopting
mailsig does not create new risks for them. He argument that this my create
some risks for their reputation in some areas, but reduces their risk in
others may carry some weight but in my experience is a pretty hard sell.
All of the mechanisms that have been discussed allow the sender to take
the harder stance that you are describing, i.e., not allow things to be
appended to the body. Other domains might want to be more permissive
than the financial service companies you mention, and might be more
interested in having the message (possibly) survive than strictly
prohibit modification.
Some domains might want to take an even stronger stance, not to allow
re-signing my mailing lists (or others using Sender to pose as one).
Otherwise it's too easy to spoof the From address, and sign on behalf of
a throwaway Sender address that most people won't look at anyway.
-Jim