Gmail does not send a notification to the blocked sender. Their emails simply appear to send successfully, but they never reach your inbox.
If you no longer have an email from them, use the central blocked list. Note that the full list is typically only accessible via the Gmail desktop site. YouTube·Krause Media unblock sender gmail
To unblock a sender in Gmail, you can either do so directly from an email they sent or through your account settings, which is useful if you have already deleted their messages. When you unblock someone, their future emails will once again arrive in your primary inbox instead of being diverted to your Spam folder. Method 1: Using a Previous Email (Desktop & Mobile) Gmail does not send a notification to the blocked sender