In the process of migrating from SharePoint 2010 to 2016 and ran into a small problem.

When trying to get the email property from the SPUser class, it returned a value of domain\userName. Clearly, this is not correct and caused some other issues.

Sample code

$web = Get-SPWeb "https://webapp.taco/toppings/cheese"
$userEnsure = $web.EnsureUser("domain\yourNameHere")
write-host $userEnsure.Email

Running this returned domain\yourNameHere, when it should have returned yourname@domain.com.
Navigate to Central Admin, then cruise over to your User Profile Service. Once there, run a full synchronization.

Profile Service –> Synchronization –> Start Profile Synchronization –> Start Full Synchronization
Run the PowerShell script again, and it will return the correct data.
Same idea as above but using the SharePoint ClientContext.
            using (ClientContext clientContext = new ClientContext("https://webapp.taco/toppings/cheese"))
            {
                Web web = clientContext.Web;

                clientContext.Load(web);
                clientContext.Load(web.CurrentUser);
                clientContext.ExecuteQuery();

                var userEmail = web.CurrentUser.Email;
           }