Software will stop being buggy as soon as people stop putting up with it.

If people actually stopped buying Windows because it sucks, you can bet your sweet darned bippie Microsoft would stop making it suck. As it is, honestly, why should they care? People keep using Windows. It makes no business sense for them to focus on quality if quality doesn't sell.

<flamebait>There is already a company that caters to the niche market that actually gives a rat's ass about consumer software quality. It's Apple -- and oh, look at how they just dominate the desktop computer market....</flamebait>

I'd agree with the comment of looking at outsourcing.
There are excellent email providers which could provide customized email solution that can easily scale to your need.
Should that not be an option, one solution to scaled to that size for me in the past is the iPlanet Messaging Server from Sun. It is also used by a lot of ISPs for their customers. Very versatile, customizeable and solid.

Good luck