> Is there an add-on for Squirrelmail where the user can schedule an e-mail
> being sent in future (ie. next Tuesday or April 10)?
SquirrelMail is only active for the short period of time it takes to
generate a webpage. You need some kind of daemon on the server side,
that's always online, for this to happen. If you find such a solution (I
haven't been looking myself) it's most likely possible to write a
SquirrelMail plugin to use it.