This article outlines common the common issues which result in an legitimate emails being flagged as spam. When you send an email, it can traverse through many firewalls and spam filters before arriving at its destination. These systems scan the emails to determine if the email is legitimate or if the email is spam. While most systems are 99.5% effective, they can also flag legitimate emails as spam (known as a “false positive”) if it fails some of the internal tests or has the hallmarks of a spam email.

To avoid your email being flagged as spam, you can use the guidelines below. We also have an article on Optimising Email Deliverability, which covers the server configuration and also very important to follow.


  1. The first thing that a e-mail is checked for is if the sender’s IP is on a block list, you can use the MX Toolbox tools: https://mxtoolbox.com/domain/
    formatting an email to reduce it's likelihood of being marked as spam.
  2. Send from your own domain, not a free or generic service such as Gmail, AOL, Bigpond or Hotmail.
  3. Format the email so that it contains many lines of content and not just one long continuous line of text.
  4. If you are sending any attachments, try to avoid sending Microsoft Word and Excel documents as their Macro Language allows malicious code to attach itself, instead try to send PDF files instead.  
  5. Make sure your e-mails don’t have a blank subject line and avoid graphic symbols in the subject line as well.
  6. Avoid embedding text inside of images or sending e-mails that are all-image, no text. An e-mail with just an image is most likely going to be blocked. Natural English text with image attachments will be a safer option.
  7. Confirm you have a valid sender protection setup such as:
    • SPF: Adds the IP addresses allowed to send email via your domain name in your DNS record. We have a guide on SPF for more detail.
    • DKIM: Also based on the DNS record, this authentication solution is used to sign e-mails with a public or private key system.
    • DMARC: This new technology allows the sender to check whether e-mails have been properly authenticated by the ISP or Webmail.
  8. Confirm Reverse DNS is setup on your IP, if you are sending from  your local office (eg a local Exchange server).
  9.  Add a signature to the bottom of your e-mail.
  10. SPAM filters will check the URL’s that you have embedded in your e-mail. If you link to a domain that has a poor reputation you will be penalised or blocked completely.
  11. Avoid using URL tracking systems which obfuscate the link.
  12. Avoid “buzz” words like the word “free” too many times.
  13. Don’t use ALL CAPS in any part of the email.
  14. Avoid using special characters designed to break up words or phrases such as Fr3e, H4x0rs or W!nn@r.
  15. Don’t use lots of coloured fonts.
  16. Only use one exclamation point at a time!
  17. Construct your e-mails using natural English language and correct grammar.
  18. Avoid using marketing phrases repetitively and avoid words commonly found in SPAM e-mails such as: “Viagra”, “Drugs”, “Free”, “Porn”, “Stocks” and “Guaranteed winner” and anything else that will trigger a quarantine.
  19. If your e-mails are too large in size (Over 5MB) you can run into issues where the receiving party won’t accept the email.

If you are sending bulk e-mails it is best to send them through a mailing service such as Mailchimp – https://www.conetix.com.au/support/article/bulk-email-sending

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