Troubleshooting Tip: how to fix Gmail response 'Gmail has detected that this message is likely 550-5.7.1 suspicious due to the very low reputation of the sending domain.' In FortiMail

In the FortiMail log, there is a response:

to=<xxxx@gmail.com>, delay=00:00:05, xdelay=00:00:05, mailer=esmtp, pri=121439, relay= [172.253.118.27], dsn=5.0.0, stat=Service unavailable(Reason from remote:550-5.7.1 [103.9.190.11 19] Gmail has detected that this message is likely 550-5.7.1 suspicious due to the very low reputation of the sending domain. To 550-5.7.1 best protect our users from spam, the message has been blocked. For 550-5.7.1 more information, go to 550 5.7.1 https://support.google.com/mail/answer/188131 41be03b00d2f7-7c3dbec0823si2768860a12.694 - gsmtp) 

To address the issue where Gmail has blocked a message due to the low reputation of the sending domain, follow these steps:

1. Check the reputation of the sending domain: Verify if the sending domain has a poor reputation by checking it against various reputation services.
2. Improve the sending domain reputation: Take actions to improve the reputation of the sending domain, such as implementing proper email authentication methods like SPF, DKIM, and DMARC, ensuring a clean sending history, and avoiding sending spam or suspicious emails.
3. Request a review from Gmail: If the message was wrongly blocked, request a review from Gmail by following the instructions provided in the error message or visiting the link: https://support.google.com/mail/answer/188131