How does hosting it yourself protect you from these issues (downtime)? The only thing you gain is control over the situation, which may be a worse prospect if you don't have the same level of resources that Google/whoever has to fix it.
I keep my cell phone next to my ear when I go to sleep. It is set to send out a piercing whistle I get an email from a particular address, which indicates that my website has been unavailable for 15 minutes. That whistle has gone off exactly once. (Credit more to Slicehost and my software stack than to me.)
You know who got blasted out of sleep at 3 AM because my email was down? I don't know, but it sure as heck wasn't me.
Luckly half our ops team is in a timezone 9hrs offset from mine, it is extremely rare for either team to have someone woken up in the middle of the night. Sleep is good.
Furthermore, it's so much cheaper/less labor intensive to let Google host your mail. How many important things is your business not doing because your technical folk are futzing with an email server? Errors of commission are easier to identify than errors of omission. Did your business fail to identify a way to attract new customers? Nobody's tracking that, but they love to count up the minutes that email's down.
