Andreas I am using the spamexperts solution for our plesk servers with around 20k domains.
It's a great gateway for in- and outgoing mails + archive, so the mailserver load for checking mails is very low because most of the spam is blocked in the gateway with a great analysis tool to check what's happened with the email and you don't have to monitor all the server ip addresses for blacklisting entries. If you have a few hundred servers it's easier to protect around 8-16 outgoing ip's with a good reputation and it's easier to switch or disable a listed ip for a while.
the price of mail.baby is ok but it's a total different thing because mail filtering is done by a third party (non european) company so that's really a problem here in the EU and I don't know what will happen if some hacked websites send so many spammails out. For us our gateway blocked 17 million outgoing spammails this year from 45 millions in total with activated max mails per hour on every plesk server. So without that it would be much more.
