Over 10% of all LISTSERV mail goes to AOL. That's a whole bunch of mail,
all going to ONE host. If your list is on a server that delivers 200k
messages daily, there will be some 20k messages a day waiting for their
turn to be sent to AOL.COM. While there is usually some measure of
parallelism, even with 10 concurrent deliveries it's a pipe of 2k
deliveries. If your delivery system isn't blazing fast, this can very
well take several hours.
Eric