LenoxSoft has a Marketing Cloud Account Engagement form on their website that adds prospects to...

Send an autoresponder to prospects who complete the form with a link to click to confirm opt-in status.

Allow the assigned sales rep to call prospects upon form submission to confirm their email address.

Edit the completion action on the form to only add to the list if the email address is valid.

Use the list to send a permission pass email and remove all hard bounces.