Validating user email ID during sign up We are making a accounting software and would like to know how we can use a open source API  or query to validate user email id by sending email to the specific email id and getting a activation response from the same using a weblink.
There's no need to use any sort of API or query. If you're goal is to validate email, it's built into Drupal's core account settings (Configuration > People > Account settings). Just make sure the Require e-mail verification... checkbox is ticked on the form page.