We already asked you for the logs - please provide them.
Validate method - as documentation clearly states - performs several checks:
- it validates email format,
- checks the existence of MX record,
- finally it checks if SMTP connection can be established and if server accepts the email to the checked address.
Most likely the last part is not working for you.
Probably the target SMTP server is refusing connections from your IP address. You can use AddressValidator.From to use specific email address/domain (so your IP passes SPF checks).
Please use ValidateMX method if you want to validate format and MX record existence only (this is probably what most other services do).