The process involved in the subscription is called two-step authentication. This is done to prevent spam bots from flooding your form inbox. If you are using a standard form builder or one within a page-builder you may not have the option of turning the two-step off. For most builders that do have it, look for a checkbox setting on the submit button (or privacy settings, etc).
The other side of the coin is that most who take the time to authenticate with you are saying, "I really want to be on your mailing list". Engage these folks immediatley! Many hit the subscribe once and disappear. Most of those are casually interested. Perhaps use a form-tracker that reminds the user to authenticate... Anyways, take a bit of time to learn about the best habits for email marketing. MailChimp, Constant Contact, Send in Blue among others all have some great tutorials.