How an address check is decided.

The sequence

Syntax rejects addresses that are not mail-shaped. The domain must resolve. MX records must exist, otherwise there is nowhere to deliver. A disposable domain or a role address is flagged. The SMTP probe says hello, starts a message, and names the recipient, then stops. It does not send the campaign.

Some providers accept every recipient at this stage. Those results stay unknown or risky rather than being marked valid. Treat them as a reason to look, not as a clean bill.