Validate mail-server TLS for a custom domain

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This answer covers “Validate mail-server TLS for a custom domain”. Read it with the summary and confirm that the symptom and service match what you are experiencing. Available options can vary by plan and account setup.

• Before changing anything, note what you expected, what happened and roughly when. Check whether the issue affects one page or the whole service.
• Save the full error message and note recent changes. Hide passwords, access codes, personal data and keys before sharing screenshots or logs.
• If a step requires administrator access, restarting services or editing server files, stop if you do not administer that server. Send support the service name, error and checks you have already made.