As stated here, the jars mail-1.4.1.jar and activation-1.1.1.jar have to be moved from from the <jira-application-dir>/WEB-INF/lib/ subdirectory of the JIRA application installation directory into the the lib/ subdirectory of the JIRA installation directory.
However, the docs seem to be incorrect, as my standard Jira installation doesn't have a <span>mail-1.4.1.jar anywhere. Similarly named files I find are:</span>
<span>WEB-INF/lib/atlassian-mail-2.8.6.jar</span><span>WEB-INF/lib/javax.mail-1.5.6.jar</span><span>WEB-INF/lib/javax.mail-api-1.5.6.jar</span>
<span>Is one of these to be moved? Or all of them? Or none at all?</span>
<span>Thanks!</span>