Many forums packages have it built-in. When someone replies to the post, the forum checks to see if anyone needs to be notified of it, and sends out the email if necessary, for instance.
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This can be mega high load though - what your host describes is notification of favorites - Threads does this, when a post is made, it emails everyone who had the post tagged as their favorites and had the email preference enabled.
What you need is the cron job which runs nightly, finds ALL the posts made that day, in a particular forum, and emails it to people that have subscribed to that particular forum.
I'm in shock by your webhost's statement. If they honestly say nobody has ever asked for a cron job, then you seriously should find a new webhost. They haven't been in the business long, or don't know what they are doing. Cron jobs are a fundamental building block of web hosting. They've been around forever.
Remember I say this not just as a webhost, but also a consultant who has hundreds of clients, all with different servers and webhosts. I've seen a pretty good cross section of web hosts.
A good webhost will offer a decent control panel (like CPanel) where you can create and schedule these jobs automatically.
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