Burnley,
Many servers work this way (plesk servers with default settings does this):
when a customer sends an email, that email is sent as if it was the end user's IP, instead the server IP (don't ask me why, ask parallels).
example: all emails that are sent though my server, are using their local own IP, except if they use webmail or web forms, then the email is "from" the servers IP
so, maybe you do live in a perfect utopian world or country, where everyone who sends email has a nice clean static IP. but I am telling you, that is NOT the case here in Spain. far from it!
not only do private homes and 3G mobile ISP use crappy dynamic blacklisted IPs, but most of the small business have dynamic IPs too!
and then I get calls telling me "hey why did that customer/provider of mine reject my email"? and i check it and have to tell em "because you reseted your router this weekend and the new IP you have is on blacklists all over the world, stop being cheap and get a static IP" which costs 6-15€/month depending on provider and that's money small business here just don't want to pay. in case you didn't know, there's an economic crisis going on.
I can only control what's going on on my server, but there are way too many other servers that are wrongly configured or have blacklisted IPs. and the communication from server to server is on port 25 afaik, and blackhole lists apply there.
example:
1) someone with dynamic, blacklisted IP sends an email.
2) his server accept it and relays it with the dynamic, blacklisted IP as source
3) the email reaches my SMTP server on port 25
4) with blackhole on, this email is LOST
(***)
Which is unacceptable
I am using plesk for around 10 years now. I am certainly no internal expert like Abdi and I might not know 100% of all the stuff... but I can tell you that I have extensive experience on what I can and what I can NOT do, having a customer-base in a country that ourselves consider "pais de chichinabo tercer mundista de mie<self-censored>" (basically "bullcrap")
And losing one single email because of using blackholes is NOT acceptable. thx to greylisting and spamassassin spam that ends up in the final mailbox is less than 0,5%
(***)
If you can tell me how to configure my server, to enable blackhole lists and not lose a SINGLE VALID EMAIL from senders that might be in a blacklist because their server sucks, their ISP sucks or whatever and without having to call my customers and reconfigure over 1000 iPhones and Outlooks manually to change a port. please be my guest, enlighten me and I will apply it ASAP.