A web digest is an automated periodic email, that users can subscribe to, that give them a summary of news, events, and forum posts on your website. It is most prevelent with forum/blog sites, as this feature is included automatically.
The user can select daily, weekly or monthly digests, to ensure they only receive as much email as they want. They can unsubscribe at anytime.
As spam is a common attack on websites, it is important that we don't continue to send digest emails out to spammers, or invalid, or stale addresses. Hence we only send email digests while the receipient is an active user of the website, or for 6 months maximum. If they have not participated online for 4 months, then we continue to send digests for 2 more months, but include a message "WARNING: Your web digest subscription will end within 60 days. Please click here to extend/modify your subscription". If they click that link, their subscription will renew for another 6 months.