What kind of billing or client management system do you use? I am using WHMCS and it has a built in function just to do that.
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Whats the platform? VPS? Dedicated? You may want to consider a 3rd party depending on how much you want to send out, read this: https://secure.mediacatch.com/suppor...barticleid=109
If its a small opertation, WHMCS does the job, windows platform? Try Smartermail5 its fast and easy.
Both really awesome mailing services. The problem with emailing off your own service is that eventually it will get black listed because it is common for people to SPAM from your server. Luckily, these solutions take SPAM real seriously because it effects the delivery rate.
If you are looking for a free software your best bet is PhpList. Spend 5 hours understanding and setting up custom features and layout and you are set your email marketing.
Now if you are looking for a paid solution then you might want to look up for solution provided by interspire. In my experience, they are not only good at making the script, but are also pretty good with support.
I would suggest trying a service like VerticalResponse if you don't think people will complain too much. VR has a great delivery rate because of their strict policies: each email campaign is approved by an account manager before it can get sent out, and they follow up really well on complaints and will quickly shut you down if you're spamming.