The "pre Windows 2000" name i.e. DOMAIN\SomeBody, the Somebody portion is known as sAMAccountName.
So try:
using(DirectoryEntry de = new DirectoryEntry("LDAP://MyDomainController"))
{
using(DirectorySearcher adSearch = new DirectorySearcher(de))
{
adSearch.Filter = "(sAMAccountName=someuser)";
SearchResult adSearchResult = adSearch.FindOne();
}
}
[email protected] is the UserPrincipalName, but it isn't a required field.