[Exim] Comments on rejecting recipients?

Hi,

What is the list's feeling of rejecting recipients at time of receipt
versus sending bounce messages? Specifically, I'm interested in
larger Exim sites that handle a lot of mail.

Both Yahoo and AOL seem to now send bounces instead of immediate
rejection of invalid addresses, which is somewhat surprising, as I
believe they both used to do immediate rejections.

Immediate rejections help reduce queue sizes, prevent bounce messages
from clogging up outbound queues and hopefully allow spammers to clean
up their lists.

Bounce rejections speed up SMTP connections, allow finer control of
CPU resources (since queue management can be used), and prevents
spammers from doing dictionary attacks.

Any insight as to why AOL/Yahoo do their mail the way they do? I would
assume that they must know what they're doing given the sheer scale
of their operations, and this issue must have been a hot topic at their
organizations in the past.

--Dean