A strong argument for using 3rd party hosted systems for email is the amount of time organisations save because the no longer have to worry about handling maintenance, software and hardware of the spam filter.

SPAM! You hear about it all the time. But what exactly is it?
SPAM is ANY unsolicited or unwanted message received via email. It is untargeted, and sent to people who have expressed no interest in receiving info on your product or service.
When a SPAM complaint is filed, it is up to the ISP and web-hosting [...]