Hello Gaby,
unfortunately that is nothing the ESAs provide, mail scheduling, priorisation, etc., main reason for this is that the appliances acting as gateways, and not so much as storage spaces. So in case this is outbound mail you are talking about, it should be configurable on the mailserver.
However, a possible workaround I could think of would be to set up a dedicated quarantine, with a release time of lets say 12 hours, and then use a content filter redirecting large messages to this quarantine. This, of course, works only for messages arriving during daytime, as the filter is working 24 hours a day.
And BTW, be aware that people easily get confused when their messages do not arrive withing five minutes. Delaying large messages could cause an even worse effect by people resending these large messages multiple times, because they think they got lost somewhere. Which would easily multiply your bandwith consumption.
Hope that helps,
Andreas
