Avoid sending bulk mails to email addresses that are no longer valid or to email addresses that you do not know. If too many people mark your mail as spam, your email address will be flagged for spamming and could be deactivated as a result. If you send out a newsletter, make sure that you provide an opt-out option to the recipients.
You should also make sure that your message includes a subject line as well as a paragraph that you wrote yourself in the email body. Review any links you might have in the email that could be considered suspicious, e.g. links that are part of a forwarded message and you did not add to the email yourself.
Spamhaus is an organization that tracks email spammers and spam-related activity by compiling anti-spam lists. It is mainly used by internet service providers and email servers to reduce the amount of spam that reaches their users.
If your IP address is "blacklisted" by Spamhaus, your emails will be rejected if the domain you are trying to contact uses Spamhaus. The best way to solve this is by contacting your internet service provider (ISP) to request the de-listing of the IP address in question.
If the issue still persists after you have requested de-listing, you can contact us and provide us with a recent bounce message and the body of the email you tried to send so we can further investigate why the email was identified as spam.