<xml>
  <customer name="Max">
    <email address="me@you.com" />
  </customer>
  <customer name="Erik">
    <email address="erik@your-mom.com" />
  </customer>
  <customer name="Brent">
    <email address="brentcom" />
  </customer>
</xml>

<customer name="Brent">
  <email address="brentcom" />
</customer>

DECLARE @x XML = '<xml>
<customer name="Max"><email address="me@you.com" /></customer>
<customer name="Erik"><email address="erik@your-mom.com" /></customer>
<customer name="Brent"><email address="brentcom" /></customer>
</xml>';

SELECT WithValidEmail = @x.query('/xml/customer/email[contains(@address, "@")]')
    , WithInvalidEmail = @x.query('/xml/customer/email[contains(@address, "@")] = False');

╔═══════════════════════════════════════╦══════════════════╗
║            WithValidEmail             ║ WithInvalidEmail ║
╠═══════════════════════════════════════╬══════════════════╣
║ <email address="me@you.com" />        ║                  ║
║ <email address="erik@your-mom.com" /> ║ false            ║
╚═══════════════════════════════════════╩══════════════════╝

SELECT WithInValidEmail = @x.query('/xml/customer/email')
WHERE @x.exist('/xml/customer/email[contains(@address, "@")]') = 0;

╔══════════════════╗
║ WithInValidEmail ║
╚══════════════════╝
    (no results)

DECLARE @x XML
    = '<xml>
<customer name="Max"><email address="mvernon@mvct.com" /></customer>
<customer name="Erik"><email address="erik@your-mom.com" /></customer>
<customer name="Brent"><email address="brentcom" /></customer>
</xml>';


SELECT x.c.value('@address', 'VARCHAR(100)') AS [email]
FROM   @x.nodes('/xml/customer/email') AS x(c)
WHERE  x.c.exist('@address[contains(., "@")]') = 0;