(Sorry, the last copy of this was sent prematurely, so I'm sending a
more complete version. Why does any MUA send a message just from
hitting Enter? Who on what planet decided that was a good design?)
A few more concerns in addition to those I expressed earlier:
- Most MUAs and most other programs that print email messages (say for
use as evidence in court cases) typically won't display or print
Expires: header fields because that field has heretofore not been
regarded as significant.
(The reason I'm aware of this is that I've sometimes had to explain to
courts why a message that's been printed out is not an accurate
representation of the message that was actually sent or received,
because the program that printed out the message tried to be too clever,
say by converting the original date timezone to a different format. )
- Messages that expire before likely delivery, or only a very short time
after likely delivery. It seems possible to use this in misleading
ways so that (for example) the sender obtains evidence of delivery of a
message (by any of several means) but the recipient never had a chance
to see it. An implementation could try to fix this, say, by always
waiting until N hours or days after receipt of a message before deleting
it, but see below.
- The more I think about this, the more I think that Expires should
never be used to automatically delete a message, even if the recipient
explicitly enables this behavior. In general, it's a Bad Idea for the
email system to destroy messages without the recipient being aware that
it's doing so. (Of course, nothing stops the recipient from writing a
sieve or other script to delete a message based on any pattern at all,
but maybe MUAs shouldn't tempt users to automatically delete expired
messages by making it seem like a normal feature.)
Expires could still be useful if the recipient's MUA displayed expired
messages differently than normal messages, thereby alerting the
recipient that the content is no longer timely, without actually
deleting the message.
Keith
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