When an email you sent bounces with Error 550 Message Rejected (or blocked), it means a spam filter has blocked the message. That is because your email domain is listed in what’s called a Real-time Blackhole List (RBL). In such cases, you will have to take action to have your email whitelisted as non-spam.
To see whether your email address is blacklisted, check your domain name using MXToolbox. MXToolbox will scan your email domain against over 100 blacklists.
If your domain is blacklisted, you must contact our support team immediately and we will request a delisting from the RBL. Please be aware that delisting takes time, so be patient.
Email domains are often blacklisted due to malware that has been installed on your computer without your knowledge. Run a malware check and removal app such as Malwarebytes to verify your computer is clean and healthy. Keep your anti-virus software up to date, make sure your computer has the latest security updates, and change your email password just in case.
You can also get blacklisted when your company’s web application is exploited through malware or a virus and used to send spam email. When that happens, your web app sends out spam using your domain name and the domain name itself gets blacklisted.
Here’s what you can do to prevent hackers from hijacking your web app: