If a contact has been tagged 'risky', that means we've checked their email address and detected that it is most likely no longer valid. If you send a Campayn email, any contacts that are tagged 'risky' in your contact list will not be sent to because there is a high chance the email will bounce.
If you know for a fact that this contact is not 'risky' and their email address is valid, you can verify their email address and untag them as 'risky' by going going to the contact Edit page for that contact, and clicking on the “Verify email” button next to the contacts email address.