From another great article by Stefan Pollard:
Even though a content filter might not punish your message as possible spam, the reader's "mind filter" might. That in turn will affect how content filters treat your messages in the future.
So it remains important to get on top of your content and understand what triggers a spam report, even if you e-mail only to a permission-based mailing list. Use this short list to help reduce the chances that your next message will be marked as spam:
- Leave no doubts in the inbox. This is your first make-or-break checkpoint. It drives the spam-or-legit decision and sets your reader's expectations for what she will find in the message. You have two chances:
- Brand the sender, or "from," line to make it clear who is sending the e-mail. Never use a person's name if it's not part of the brand or company name.
- Use a clear subject that accurately sums up the subject line. No vague promises or hints about the content here.
- Spam-check copy before sending. True, I did just say that content filters don't always catch what a mind filter would interpret as spam. However, many of them are based on what others have reported as spam, so the probability is high that a prelaunch check will highlight items that need to be corrected.
- Use a deliverability-monitoring service. Your e-mail service provider (ESP) might already have one of these as a contract or add-on service, or you can investigate the best-known third-party services: EmailAdvisor, Return Path, and Pivotal Veracity. They also scan content before sending and can predict how ISPs will treat your e-mail: whether it will land in the inbox or the junk folder, or get blocked.
- Design for the preview pane and blocked images. If your readers see just blank space or a bunch of red Xs where pictures should go, they'll more likely suspect it and mark it as spam.
- Respect frequency and content preferences. A strong inbox presence and utter lack of spam signatures won't help you if you mail-bomb your list with irrelevant messages. Remember: people click the "report spam" button deliberately if they feel you're abusing the privilege.