Validating XML against XSD containing xsd:import without location

The XML schema for http://www.w3.org/XML/1998/namespace namespace is located here: https://www.w3.org/2009/01/xml.xsd

So, you can just specify its location in <xs:import> in your schema:

<xs:schema xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types
    xmlns:tns="http://schemas.microsoft.com/exchange/services/2006/types"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
    targetNamespace="http://schemas.microsoft.com/exchange/services/2006/types"
    elementFormDefault="qualified" version="Exchange2010_SP2" id="types">

    <xs:import namespace="http://www.w3.org/XML/1998/namespace" 
               schemaLocation="https://www.w3.org/2009/01/xml.xsd"/>
...

That will work, but note that W3C doesn't like huge traffic to that file: http://www.w3.org/2001/xml.xsd. So, they delay artificially the access to it.

Many software hold local copies of such schemas. (That's why the schema location is not specified. The schema software typically loads it from its resources).

You may also copy it to you computer and specify the URL to that copy.

An alternative way is to use XML catalog, like this (catalog.xml):

<?xml version="1.0"?>
<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog">
  <!-- 
    This will redirect the namespace URI to the local schema file,
    which should be found in the same directory as the catalog.xml
  -->
  <uri name="http://www.w3.org/XML/1998/namespace" uri="xml.xsd"/>
</catalog>

But you will have to pass somehow that catalog file to your schema processor software (if it supports XML catalogs)