I strongly suspect (though don't have hard data to prove it) that they are quarantining emails for AV scanning before letting them go on to the intended recipient. It is probably done in "batches" or increments on certain servers, based on a schedule to try to "load level" their systems. Back in the old days of the internet, many systems were interconnected via more restrictive communications links, so many systems were "store and forward", meaning that they would store all incoming email, and forward it onto the next link in the system at prescribed times .
While Yahoo may do something about virus scanning, etc. they don't seem to do anything about SPAM messages as most of my SPAM seems to still come from Yahoo systems, or other systems that are spoofing the yahoo domain...
