It is essential to monitor your subdomains to ensure that are all configured properly to work with Adobe Campaign.
Subdomains & Certificates
Last verification Verify subdomain
Adobe does not recommend using subdomains with no certificate date as it could mean that these subdomains may be having some deliverability issues.
When launching a verification, several operations are performed to check that the subdomain is correctly configured (instance tenant check, email sending test, etc.)
If the subdomain's verification fails, contact Adobe Customer Care for further investigation.