Kindly login to your dashboard using your http://reseller_url/webadmin/ and go to System Settings -> Signup Activation as show below.
a) Mail will only send the mail verification code notification.
b) Sms will send only sms for about the verification code.
c) Both with send mail as well as sms to the signup users.
Note :- If you have selected the Gmail for signup activation then you will get the Alert to your inbox for the suspicious account just re-set your gmail password and applied the same password in the under the admin panel System Settings -> Signup Activation where you have selected the gmail.