Better bulk email solution

Hi,

I know this looks a bit long winded but it's just advice I'm after and you get a nice 500 points :).

I work for a company that designs, builds and hosts web sites for our clients. Some of these clients want to send personalized newsletters to their users. Personalized in that there are certain 'subjects' that users can elect to receive news upon. So, for example, one user may have elected to receive news on 'Technology' and 'Art', while another may want 'technology' and 'Marketting' etc. The system then generates the newsletter for a particular user, with the relevant content and mails it out to them. We have no problem generating these personalized emails, but we are struggling with an efficient way to send these emails out on mass. We do have our own SMTP server and we are an ISP so there are no limits to worry about. What I am worried about is clogging up our bandwidth and getting blacklisted as spammers - which we're not, and neither are our clients. We don't have the resource to monitor and moderate the sending of our client's emails to ensure we're not seen as spammers.

I have looked in to third party email solutions like Constant Contact but a major limitation for us seems to be that you can't have personalised content for each user as detailed above. I also don't know if I would be able to generate the content on our websites and then send it to them for emailing. I'm looking for something that can be integrated into our sites.

Is this just a case of investing in a 3rd party SMTP service, if so can you recommend any? or is there a better option?

Thanks.