I am going to write a software to deliver newsletter email to my clients.

The program is easy to write, but I am afraid that the email server of the recipients would regard my newsletter to be spam. And the worst thing is to backlist my IP. Would this happened?

If yes, how can I aviod it? (I am quite interested in the mechanism of mailing list, does it have something done to aviod this issue.)

I'm no expert, but whenever I want to join a mailing list I do so either through a web interface or by sending an email with "subscribe" in the subject line. That means I take the initiative and make the choice. If a company just started sending offers or product updates to me without my authorization, it is just going to end up being marked as SPAM and I will see no more of it. So if you have a website, just add an interface that will allow clients to subscribe to your mailing list or include the option in your e-mail signature when you correspond with your clients via e-mail (i.e. when you reply to their e-mails). I think that would be the polite way of doing it, but like I said: I'm no expert.
And if you think about your question, I'm afraid it looks a little bit like the sort of question that could be asked by a spammer looking for a way to avoid being marked as a spammer. I'm sure you have no such intentions though.

If your clients agreed to having emails sent to their addresses for offers, promotions, info, etc.. then it shouldn't get marked as spam. If they don't have any idea that your going to start sending them these emails, it's unsolicited email, which is called spam and you should be blacklisted by those who choose to not want your email.

if your legitamet email gets marked as spam, its their problem not yours. I would suggest adding a line at the start of all your email that doesnt change (your program could add it) that states its your newsletter to them, so if your spam filter does block it, and they want it, you can just tell them to let emails with your signature at the top/bottom get thru.

My worry is that the situation may not be controlled by the clients.
For example, a client registered my newsletter by a Yahoo email account. If yahoo regarded my email as spam(since my newsletter may be very commerical and the spam-filter may detect those backlisted wordings), the client can't control yahoo to un-filter my newsletter.

May be yahoo/hotmail or other public emails are better since they also stored the spam in a different folder in your account. But some companys(at least my company) will block the spam and don't alert the recipients.

JunctaJuvant, you are right, I am looking for some ways to "avoid being marked as a spammer", of course, I am not a "SPAMMER". :-)