Information
If this setting is enabled then you will not be able to configure mutual authentication TLS, referred to as 'External send connector authentication: Domain Security' in this baseline.
Rationale:
Basic authentication sends credentials across the network in plaintext. TLS helps protect credentials from interception by unauthorized users.
Solution
To implement the recommended state, execute the following PowerShell cmdlet:
set-SendConnector -identity -IgnoreSTARTTLS: $false