There are a lot of options out there if you want to do e-mailing. You can pay for a hosted (pay for use or SaaS) service like MAILCast, or you can download and install one of the milliard desktop bulk mailing programs that exist. In this case you even have several available for free.
You must choose wisely.
Choosing between a hosted service and a desktop program is like choosing between connecting to the electric grid and having your own in-house power plant. Yes, sure, you can have your own generator and work with the energy it produces. You even will have more sense of having the control.
But it's not true and you surely can have a lot of trouble too. What if the small power plant get damaged on weekend?, What about the complexities of plugging it to your home's grid so that you don't get any of your small appliances fried? You could spot a lot of these "gotchas" yourself...
The same is true for complex software like e-mailing applications.
First of all there is the infrastructure regarding to deliverability and speed. You cannot download an e-mailing program, install it in a "Next, Next, Next" way and expect that everything will work. It simply is not that simple (pun intended). You need to set up some DNS infrastructure so that the e-mails get correctly delivered. And, if you're going to send more than a few tens or hundreds of emails, you can't rely on using your usual external e-mail server (as several of these programs do) without having it blocked for hours. If you choose this kind of desktop product, count on having a lot of your e-mails rejected. Besides this, a lot of servers reject e-mails directly delivered from DSL connections, and the upload bandwidth you have in one of these is very low (128 or 256 Kbps if you're lucky) which leads to long delivering time spans.
Then there are the stats. You need a permanently connected computer with a static IP in order to collect stats for your e-mailing. And usually a huge database too. What if you suffer a blackout? You'll miss stats. What if you receive thousands of reading stat request in a few minutes? Repeat after me: "I will miss stats". And that's without taking in account that you probably must serve graphics too through the same connection.
A hosted service will take in account everything for you. As it's usually said "The cobbler should stick to his last". Emailing is the core of your business or it's to sell or to make campaigns for your customers? So, why are you trying to setup your own emailing infrastructure? When you need to go from London to Seattle on a business travel, do you charter your own plane? Even though you do this travel frequently?
And finally there are support and service... e-mail marketing is a fair technical discipline and you will need support, advice and a good service level.
There is a long list of issues like those in this post, but I've only reviewed the most apparent and important ones. And I don't even have touched the economic part of the decision and the amount one can save using a pay-per-use service, which would be interesting enough to deserve a whole post by itself.
Choosing a hosted service is the wiser of the options and the less risky too. Stick to your core activity and let the professionals handle the technical challenges, and keep your deliverability in shape.