When sims email on the computer...

[Cassie]

This is a great idea, it could definitely work.
But I don't think we should get to write our own e-mails, more so we should just have a reasonably large selection of pre-made e-mails that are all programmed to have different reactions, hopefully depending on the type of Sim (relationship to sender, traits, mood...). I also think users should be able to make their own e-mails on sites like MTS2 (obviously not that actual site but as an example) that if downloaded, would appear in the pre-made e-mails list. Obviously the creators of the e-mail would have to create some kind of reaction or use a pre-made reaction that fits with the e-mail.
I know this isn't as fun as writing your own, but as Chanel said, it would be too hard to program and way too complicated and would probably take more space than necessary otherwise.