Unfortunately not - the subscription e-mail cannot be sent when a user posts to a forum. The reason is this - a forum that has many subscribers would need to send an e-mail to every user when posting. So, to take our Announcement forum as an example, 108 e-mails would be sent every time someone posted a message to it. This would introduce all sorts of problems - it may appear to take a long time to post for the user posting, there are all sorts of timeout problems - there is no guarantee that the web server will let the process run long enough to send all of the e-mails, or the user might get fed up and hit Stop.
You might try asking your ISP to set up a cron job (or some other equivelant) for you anyway, explaining that it needs to run only once a day and sends e-mail to subscribers of your forums. It may be that they don't like unauthorized cron jobs running.
Jason Rhinelander
Gossamer Threads jason@gossamer-threads.com