If you get one complaint out of thousand emails sent, your dedicated IP address runs the risk of being blocked by one or more ISPs.

In this blog post, Neil Anuskiewicz shares a bunch of tips to reduce spam complaints:

1. Never harvest email addresses off of Web sites.
2. Even if it is opt-in, be careful when sending to an old list that has not heard from you in a while.
3. Do not just email every contact you have.
4. Do not purchase email lists.
5. Too many hard bounces (email addresses that do not exist any more) can get you blocked by a number of ISPs.
6. Do not send too many emails at once but ramp up.
8. Configure a Sender Policy Framework (SPF) record in your Domain Name Service (DNS) records for your domain name (company.com).
9. Make sure to do a plain text version of your email, too.
10. Very small fonts can get you filtered.
11. Consider putting the unsubscribe link at the top.
12. Maintain a good balance of graphics to text.

Read more here.