You clearly haven't run an email server in the past few years.
You are now required to utilize reverse (PTR) records on your IPs, DKIM and SPF records, run inbound and outbound SPAM filters.
Blacklists kill businesses. It is frustratingly time consuming to have to deal with getting an email from an executive with a "Fwd: Message failure, return to sender" in the subject.
Then, as an IT admin, you have to go and hunt down how to get your IP off that blacklist. Some are easier than others, some require faxing in a signed statement of what you as an admin have done to prevent the problem in the future.
Sometimes you get on blacklists because your datacenter provider has their whole /20 subnet blocked by Google or AOL because of one hacked wordpress site somewhere else in the datacenter.
Oh, but running my own from the office will solve all of that! Big fat NOPE.
Running one from your own office requires all of the above, plus segregating your email server from the rest of your internal network. Not to mention you'll need permission from your ISP because they'll need to unblock sending email from your IP(s). What happens if your email server stops responding in the middle of the night? Will you drive 2 hours to the office at 3AM to go deal with it?
$5/mo/account for Google Apps sounds really appealing all of a sudden.
(Source: I own a web hosting company)