I'd also recommend that the link point to an archived copy of the
message in original form, rather than, say, one that is translated to
HTML. Translating to HTML causes a loss of information and
potentially
a loss of functionality. You should be able to reply to an archived
message, refile it into a folder, follow threads, etc., but those
things are harder to do if the message is no longer in its original
format.
I have added some text about different formats, along the lines
suggested by Graham. I think functions such as being able to reply,...
would be great. Actually, our archives have such a function;
look for "Mail actions: [ respond to this message ]". The functionality
is limited to what can be done with the 'mailto' URI, and only
preserves some headers, and not the body. If you have ideas on
how to implement improvements, that would be great.