So are you saying you want to send the content of a tiddler to someone by email with no need to open a separate email client?
Are these occasional messages personalised for the recipient or sent in batch to many users?
If you can't make the client "tiddlywiki" do it, then you need something to do it on your behalf. A server process could respond to an exported tiddler or a request like Jed suggested or on the machine that the tiddlywiki is running could have a method that sends the email for you.
I would still prefer more info. Where are these wikis hosted?, are they single file or node wikis or can you choose? is there one for one user only?. Does this sending need to be done by one user from there own wiki or do more than one user us it on different computers?.
Perhaps multiple messages can be prepared and exported as a cave file with a mailer program then sending the message?
I think we can solve anything we just need to understand what the thing is.
Regards
Tony