• Go to Administration (Gear next to profile) > System > Logging and Profiling page
  • This can also be accessed by pressing 'g+g' and typing Logging and Profiling in the pop-up.
• For Outgoing Mail,
  
  • Scroll down the page to the Outgoing Mail Section
  • Ensure that you have an Outgoing Mail configured and that Outgoing Mail log is enabled as without it, debugging cannot be enabled.
  • Click the blue Enable link that appears in this section to enable outgoing mail debugging.
  • To persist the debug logging enabled, even after you restart Jira:
    • Edit the <jira-installation>/atlassian-jira/WEB-INF/classes/log4j.properties file.

    • Locate the section:

      log4j.logger.com.atlassian.mail = INFO, outgoingmaillog

      and change INFO to DEBUG.

  • For batch notifications specifically, set the package com.atlassian.jira.plugins.inform.batching to DEBUG
• For Incoming Mail,
  
  • Scroll down the page to the Incoming Mail Section
  • Ensure that you have an Incoming Mail configured and that Incoming Mail log is enabled as without it, debugging cannot be enabled.
  • Click the blue Enable link that appears in this section to enable Incoming mail debugging.
  • To persist the debug logging enabled, even after you restart Jira:
    • Edit the <jira-installation>/atlassian-jira/WEB-INF/classes/log4j.properties file.

    • Locate the section:

      log4j.logger.com.atlassian.mail.incoming = INFO, incomingmaillog

      and change INFO to DEBUG.

• To enable email logging for incoming and outgoing at the protocol level (more verbose), add -Dmail.debug=true to the Jira startup parameters.

For more information on the subject, please refer to Logging and Profiling.

Keep in mind that the primary goal of debug logging is troubleshooting. A good practice to avoid potential performance impact and logs with too many entries in the future is to disable the debug logging after the problem is solved.