How to give access rights to SPListItem in Sharepoint

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C#

SharePoint

Following code is use to give permission to user to access SPListItem

//Add current user
 tempRoleAssignment = new SPRoleAssignment((SPPrincipal)currentUser);
                         tempRoleAssignment.RoleDefinitionBindings.Add(web.RoleDefinitions.GetByType(SPRoleType.Contributor));
item.RoleAssignments.Add(tempRoleAssignment);

but it is not work when current user domain server is different than the sharepoint server.

More information
My Sharepoint server name is spserver. Domain server name is domainserver. When I try to give right to user of domainserver to sharepoint server SPListItem it cannot assign as I can able give right to any sharepoint group. It does not fire any error/exception by using above code but user cannot have right to access that perticular SPListItem.

I wonder if anyone can help me?

Thanks