title: X509 Certificate issue with Go smtp.SendMail
body: <p>When using Go's smtp.SendMail to send an email to support@groupsio.zendesk.com, I get the following error: </p>
  
  <p>x509: certificate is valid for mx.zendesk.com, www.mx.zendesk.com, not mail.pod-4.int.zendesk.com</p>
  
  <p>Before calling SendMail, I do an MX lookup on groupsio.zendesk.com, which returns mail.pod-4.int.zendesk.com. So, the address I pass into SendMail is mail.pod-4.int.zendesk.com:25.</p>
  
  <p>This used to work, but something broke and I can't figure out what's wrong. If I send a message to support@groupsio.zendesk.com from Gmail, it works fine.</p>