The JIRA team have discovered a bug in atlassian-mail that will affect anyone trying to set up an SMTPS mail connection via a JNDI mail resource. Basically atlassian-mail hardcodes the transport it uses to SMTP, so all attempts to set-up SMTPS fail fairly obliquely. There is a workaround, but we are not yet sure that that applies successfully to all users.

There are no specific Confluence docs on the issue and setting up SMTPS is not documented: http://confluence.atlassian.com/display/CONF256/Configuring+a+Server+for+Outgoing+Mail

The workaround given by the JIRA team is to specify an SSLSocketFactory for the resource, see the gmail config in the JIRA docs:
http://www.atlassian.com/software/jira/docs/latest/smtpconfig.html