How can I verify the legitimacy of sudden email subscribers?

[Quondy]

Hello everyone!

These last 2 days I have been receiving lots of email subscribers - from 213 I have now 259.

I have checked and most of them have subscribed on the same day at the same time. It looks so suspicious to me. 

For email subscription, I have the google recaptcha enabled. 

I have checked some of the email addresses on google and most of these "subscribers" are from USA while in the customer page they are registered as living in HK (my main selling market).

Is there a way to know if these email addresses are legit or not?

Thanks for the help!