ISP filters scan every aspect of your email to detect spam like content, including headers, subject line, content, pictures. They check where your email is coming from, and check your previous email history. It is crucial to make sure your email content won't be flagged as a potential SPAM email.
Based on the findings the email filter determine whether to deliver your email and if it should be placed in the Inbox or Spam folders.
Fortunately there are some online tools that can help you testing the spammyness of your email. The best tools on the market:
- Mail Tester
- Glock Apps
Mail Tester provides a free report on how likely your email will hit the Inbox. Ideally you should be sending your email campaign when your score is 10 / 10. If you are scored down for anything that means your email might not be delivered in Inbox or it might bounce back (soft bounce).
Mail tester checks:
- Spam Assassin score
- Authentication tests: SPF, DKIM, DMARC
- Errors in code
- IP blacklist checks
- Broken links
Litmus is not a free application but goes a lot further than Mail Tester. Along many other functionality they have newsletter builder, spam tester, email analytics tools. Their spam tester also uses Barracuda, Cloudmark, Miscosoft Exchange, Message Labs spam filers as well as Spam Assassin.
On Litmus you can also test where your email is placed on various major ESP's.
Find out more here: https://litmus.com/spam-filter-tests
To generate a report they resend your email using their own accounts. This way you can compare inbox delivery rates and see how the email content and IP reputation affect deliverability. Use GlockApps to identify risky content, spam score, authentication testing and inbox rate tracking. More: https://glockapps.com