It would be very useful to have a feature where the sending of digest mails would be spread in many days (limiting by the number of mails per day, or something similar). Lots of mail providers, usually allow you to send only a limited of mails per day, however, if you have a large user base, what happens is that a lot of these digest mails will be sent on the same day, exploding the limit of daily mails and therefore blocking other important mails such as signups/etc, not to mention that it might get caught on spam heuristics of many mail servers.
An alternative is to have an option to limit the number of digest mails that will be sent per day, that way the digest will be sent spread across the week instead of being sent at same time for a lot of users.