To configure Confluence to use Gmail to send emails, you will need to create a JNDI mail session and then have Confluence use it as per this document.

Please see Setup a mail session in standalone version for some guidelines.

Tomcat 5.5
<Resource name="mail/Session"
    auth="Container"
    type="javax.mail.Session"
    mail.smtp.host="smtp.gmail.com"
    mail.smtp.port="465"
    mail.smtp.auth="true"
    mail.smtp.user="nobody@gmail.com"
    password="foobar"
    mail.smtp.starttls.enable="true"
    mail.smtp.socketFactory.class="javax.net.ssl.SSLSocketFactory"
  />

Note: You may optionally add mail.debug=true into the <Resource> to see logs generated by JavaMail.

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