I have just inherited on old Umbraco 8 site which uses NewsletterStudio and has the following nuget packages installed:

NewsletterStudio 3.0.9, NewsletterStudio.Core 3.0.9, NewsletterStudio.Web 3.0.9

On trying to get the site to build, the 1st issue I have is with the NewsletterStudioSubscribe.cshtml, any line using, NewsletterStudio.Api, eg

if (NewsletterStudio.Api.IsValidEmail(txtEmail))

show a red line under it with the error,

'Api' does not exist in the namespace 'NewsletterStudio'

As far as I can tell this view is provided as a template and hasn't been touched. Am I missing something else I need to install?