Notifications and the Share Page Functionality do not Work in Confluence 3.5 or Later
In a 3.5+ instance that was upgraded from a pre-3.5 instance, notifications and the Share Page functionality do not work. Sending a test email from the Mail Server configuration admin page does, as do daily digest and forgotten password emails.
The following appears in the Confluence log files:
java.lang.ClassCastException: com.atlassian.mail.server.impl.SMTPMailServerImpl cannot be cast to com.atlassian.confluence.jmx.JmxSMTPMailServer
The internal storage of Mail Servers was changed in 3.5. Old mail servers should have been migrated on upgrade, but if the upgrade process does not complete properly or earlier upgrade tasks fail, the Mail Servers won't be upgraded correctly.
From the Mail Server admin screen in your 3.5 instance, delete your existing mail server and add it again with the same properties.