These errors occur when a sender attempts to start a TLS connection but does not complete the handshake and then the connection is rejected, usually due to timeout. While these errors can arise due to general network issues that cause the TLS handshake to fail, research has shown that the usual culprit is a non-valid mail sender that is not adhereing to appropriate connection protocols for closing message transactions. For the latter, this error can be ignored.
More information is available at the Information logging level:
Click Logs in the left pane.
Set Mail Transfer Agent to Information.
Click the Save button (Note: this will restart the MTA and scanning components).
With Information logging, the unhandled rejection errors are followed by an ML-HOST_DISCONNECTED log entry, which indicates the IP:port that caused the error: