Is this domain on Microsoft 365?

Check any domain’s mail provider from its DNS. Microsoft 365 is where most email tools guess and bounce — Engage doesn’t guess on those inboxes, so you don’t.

This tool reads public DNS (MX records) only. An MX behind a security gateway can still hide a Microsoft 365 tenant — the only definitive check is a per-mailbox lookup against Microsoft’s directory, which is what Engage does.