Mailbox should not use the .onmicrosoft.com domain as primary SMTP address.
Why This Matters
When a mailbox retains the default .onmicrosoft.com domain as its primary SMTP address, external email communications may appear unprofessional or suspicious to recipients. This configuration can also complicate domain management and increase the risk of email spoofing if the custom domain is not properly authenticated.
What Aether365 Checks
Aether365 scans all mailboxes in your Microsoft 365 tenant to identify any that are still using the .onmicrosoft.com domain as the primary reply-to address. This check is listed under the microsoft-365 security checks in your Aether365 dashboard.