This might help:
<a href="https:&#x2F;&#x2F;support.google.com&#x2F;mail&#x2F;answer&#x2F;21291?hl=en" rel="nofollow">https:&#x2F;&#x2F;support.google.com&#x2F;mail&#x2F;answer&#x2F;21291?hl=en</a><p><i>What are the SSL certificate authority requirements?</i><p><i>We do not accept self-signed certificates. For a certificate to be valid it needs to chain up to a valid CA, like one in the Mozilla CA list.</i>