• IP reputation

First of all, the reason for your messages to go to the junk folder can be related to the reputation of the IP address of your mail server. The IP reputation relies on many factors such as volume of sent emails, content of the messages, bounced messages, messages sent to not-existing email boxes, messages hitting spam traps. All that is used to calculate the reputation of your IP. Then mail servers decide where they will deliver the email message based on the IP reputation score.

• The destination server blocks your domain

Another reason for deliving emails to the spam folder is the fact that your domain exists in the blacklist of the destination mail server. To find out if that was the case you will have to contact the mail administrator of the destination server.

• Missing or wrongly configured reverse DNS record

You can see such an issue if your mail server is not properly configured and it doesn’t have reverse DNS or/and the hostname of the server doesn’t point to the IP used to send email messages.

• Anti-spam filters

The biggest free mail providers such as gmail, hotmail and yahoo have anti-spam filters which in many cases take decisions to deliver your non-spam message to the spam folder. It’s pitty you can’t do much about that because you can’t control what their filters do. If you contact them and ask them why they deliver your legitimate e-mails to the junk folder they will tell you about the anti-spam filters. They will not tell you the exact reason though. They provide free mail services and they can do whatever they want with the messages. Everyone uses their free services on their own risk.