Hello Francisco,
Our most common reason for this issue is not having enough physical memory. See if you can increase this and hopefully it should resolve your issue.
Otherwise, it can be a common error message under load. If you are just seeing it on occasion, it could be safely ignored.
Title: 'Error: "Failed to connect to the firewall channel" shows in the logs for Symantec Mail Security 5 for SMTP'
Thanks!
Tom