There are several ways to help avoid getting the email you send blocked including
- Send plain text email
- If you send HTML email use a high text to image ratio
- Do not send bursts of similar email to large email providers
- Verify your SPF records and DKIM signatures do not have any errors
- Do not send email to addresses that bounce with permanent 5xx SMTP error codes
- Do not send email from blacklisted IP addresses
- Personally address and resolve all spam complaints
Additional suggestions can be found in our
http://www.emailquestions.com/email-articles/2437-top-10-email-marketing-tips.html