One issue I have is as some of my clients grow their maillist, they eventually run into problems with Yahoo in which messages are deferred. To get around this I have set their domain to throttle messages to 100 / hour. But the problem is throttling effects everyone and not just just messages going to yahoo. What I would like is a feature that limits messages to X per hour per domain.
This way smaller domains, are delivered fast, while larger domains are spaced out. This would help in staying under the radar with yahoo, gmail, and a few others, that tend to not like high volumes of email from a single IP.