Summary

Upgrading Confluence recreates the mail-x.x.x.jar file that is moved when creating a JNDI Connection.

As explained on Setting Up a Mail Session for the Confluence Distribution, you need to ensure that you move (don't copy) mail-x.x.x.jar from <confluence-install>\confluence\WEB-INF\lib to <confluence-install>\lib (x.x.x. represents the version numbers on the jar files in your installation).

When you upgrade Confluence, the mail-x.x.x.jar file is recreated/kept in the installation folder. This causes issues when since email will not be sent when both files are in place.

Steps to Reproduce

1. Set up a JNDI SMTP connection
2. Upgrade Confluence manually
3. Notice the upgrade tasks do not remove the mail-x.x.x.jar

Expected Results

The upgrade should check to see if there is a JNDI connection originally and remove the mail-x.x.x.jar file

Actual Results

The below exception is thrown in the atlassian-confluence.log file:

An error has occurred with sending the test email:
java.lang.IllegalArgumentException: Mail server at location [java:comp/env/mail/GmailSMTPServer] is not of required type javax.mail.Session.

Workaround

The workaround is detailed in the guide Mail server at location is not of required type javax.mail.Session.

1. Shutdown Confluence
2. Delete the mail-x.x.x.jar from <confluence-install>\confluence\WEB-INF\lib (x.x.x. represents the version numbers on the jar files in your installation)
3. Restart Confluence and attempt to reconfigure your mail server again.