Depending on your diagnosis, apply one or more of these fixes.
This error typically surfaces when Postfix is told to use a specific "transport"—a method for moving mail—that it cannot find or verify. Missing Transport Definitions Depending on your diagnosis, apply one or more
: A completely full root filesystem can prevent Postfix from writing to its queue or reading configuration updates, causing it to suspend all delivery. Diagnostic Steps Since "unknown mail transport error" is often the Depending on your diagnosis
Send a test email. Watch for: