Get user login name by email address even if the user is still not ensured

Question

Is there a way to get the user name or the SPUser object by passing the mail address for a user, which is not already known by the site?

I know I can get the SPUser by using

SPSite.AllUsers.GetByEmail(emailAddress);

But this only works if the user is already known in the site collection. Does SharePoint (with C# Server Object Model) provide a way to search a user in the Active Directory by its mail address or do I have to use System.DirectoryServices and search in the Active Directory for myself?

Use Case:

I have a mail event receiver for a document library people can send mails to. I have some logic and at the end I want to save the attachments to this library. The Author property of the item (which is now set to SystemAccount) should be set to the person who sent the email.

Answer 1

Please check the below code.In this way you can get the login name of the user via email address.

   using(SPWeb oWeb=new SPSite("-----URL---").RootWeb)
   {
            SPList oList = oWeb.Lists["Test"];
            foreach (SPListItem i in oList.Items)
            {
                if (i["email"] != null)
                {
                    string email = Convert.ToString(i["email"]);
                    SPWebApplication webApp = oWeb.Site.WebApplication;
                    SPPrincipalInfo principal = null;
                    principal = SPUtility.ResolvePrincipal(webApp, null, email, SPPrincipalType.User, SPPrincipalSource.Windows, true);
                    if (principal != null)
                    {
                        MessageBox.Show(principal.LoginName);

                    }
                }
            }
   }