[moray]

The thing is that it also depends on the client side (meaning the software client) and from the sender behaviour.

For instance, I have a client (person) who now and then sends out a newsletter to some hundreds of addresses. He does that directly from the webmail. I always catch him because I have an alert on the mail queue, and every single time I check it I notice that the messages have been delayed because the remote server has a filter on "too many recipients at the same time" or "server sending too fast" or similar.

This affects the way the remote server will consider further emails from your server, and change its "behaviour" temporarily. I use Postgray [1] that does very similar checks, and it is a great deterrent for incoming spammers. So I completely understand this strict checks from the point of view of a sys admin.

Then there is the email content and headers that are checked by spam filters... but that's another story...

[1] http://postgrey.schweikert.ch