Which mailbox providers support MTA-STS checking?
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Here's the reality: MTA-STS adoption's still patchy. Gmail and Outlook check MTA-STS records when they receive your mail, making them the big enforcement players. Yahoo supports it but doesn't enforce it yet. Everyone else's a wildcard.
Here's what matters for you as a sender. Your email provider (the one you send from) isn't the gatekeeper. The receiving provider is. So when you send through Gmail or Outlook, Gmail's and Outlook's receivers will validate your MTA-STS policy. If you're sending from a smaller host, you need to check whether major receivers will respect your MTA-STS setup.
The move: Test your setup with an MTA-STS validator to see which providers will actually enforce it. Then layer in solid caching strategy for maximum reach.
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