We have an application that sends out emails to users of our website when a watched item changes. I'd like find a way of grouping these emails together into a digest format. Ideally rather than making the web-application store the emails and group them into digests, I'd like to find some sort of middleware that I could route all the mails to, and group them into a digest for the user if they selected that option. Do any linux based mailservers or applications have this ability?
I can't recall seeing any MTA-type software that does this out of the box.
Some mailing list managers have this capability but I don't think that would be a very good fit for what you're trying to do... you'd have to do something like create a mailing list for each user and only subscribe the user to it, then toggle the digest option.