Hi
At the moment Dovecot does not implement an SMTP/LMTP client. This might change, when Timo decides to implement all of the LEMONADE feature, which at some point require the IMAP server to edit and send messages on behalf of a (mobile) client. Timo will shed more light on his plans.
Are you thinking about burl smtp?
Someone from Apple implemented this for postfix some years back, but it doesn't seem to have made it into mainline (I think through oversight and Apple not pushing a second time though...)
It will need client support, but my design would be something like an IMAP extension which works something like "SMTP *this* specific message using these login details and these sender/recipient details".
That way the mail client can completely generate the mail using any IMAP tricks at it's disposal to minimise traffic, once the mail is generated and in some location, eg Sent, Drafts or INBOX as per your preference, then finally instructing the server to push it into the normal SMTP system (for bonus marks it could forward the clients IP using X-FORWARDED-FOR so that the SMTP can make decisions based on IP address). This design gives you all the benefits of keeping the SMTP system, minimises traffic, allows for storing Sent Items or not as per your preference and avoids the use of magic folders. Now all we need is client support...
Note there is a feature of Courier which does something similar, but it uses magic folders (ideally we want to be able to smtp any message in any folder in order that we can easily implement our preferred storage policies) http://www.courier-mta.org/imap/INSTALL.html#imapsend
Ed W