18.104.22.168. Error "550 5.7.1 This message was blocked because its content presents a potential security issue"
The error can occur when sending emails with attachments containing malicious code. You can check the presence of malicious code in the attached files, for example, using the service VirusTotal.
If the attachments are indeed infected, you can:
- Clean up the forwarded files with antivirus software and send the letter again with them.
- Upload files to any file sharing service and send only a link to the file instead of the file itself.