outllok 2007 breaks signed email???

[Phillip Drummond]

if i send a digitally signed email in html format using outlook 2007, when
the receiver reads the message using windows mail (vista) they are alerted
that "the message has been tampered with"

if i send the message as plain text, they do not get the warning.

i found a kb article that suggests that if the message is sent with the
option to "break apart large messages" this will cause the issue, but
outlook 2007 has had that option removed. the only thing i can find that i
thought might alter the message is the option to "reduce email size by
removing unnecessary formatting data", but even with this unchecked the
warning is still displayed.

can someone please shed some light on this? i need to send signed html email
without people thinking the message was altered

thanks