There was an error loading the descriptor 'Create a new issue or add a comment to an existing issue' of plugin 'com.atlassian.jira.jira-mail-plugin'. Disabling.

[Sathish Kumar Venkataraman]

I upgraded JIRA from 5.x to latest 7.x.  I see this error.  any help on what is the resolution?

2016-06-03 18:31:44,844 JIRA-Bootstrap ERROR      [c.a.plugin.manager.DefaultPluginManager] There was an error loading the descriptor 'Create a new issue or add a comment to an existing issue' of plugin 'com.atlassian.jira.jira-mail-plugin'. Disabling. java.lang.ClassCastException: com.atlassian.jira.plugins.mail.extensions.ProjectRequiredMessageHandlerValidator cannot be cast to com.atlassian.jira.plugins.mail.extensions.MessageHandlerValidator  at com.atlassian.jira.plugins.mail.extensions.MessageHandlerModuleDescriptor.enabled(MessageHandlerModuleDescriptor.java:58)  at com.atlassian.plugin.manager.DefaultPluginManager.notifyModuleEnabled(DefaultPluginManager.java:1991)  at com.atlassian.plugin.manager.DefaultPluginManager.enableConfiguredPluginModule(DefaultPluginManager.java:1738)  at com.atlassian.plugin.manager.DefaultPluginManager.enableConfiguredPluginModules(DefaultPluginManager.java:1715)  at com.atlassian.plugin.manager.DefaultPluginManager.enableDependentPlugins(DefaultPluginManager.java:1227)  at com.atlassian.plugin.manager.DefaultPluginManager.addPlugins(DefaultPluginManager.java:1188)  at com.atlassian.jira.plugin.JiraPluginManager.addPlugins(JiraPluginManager.java:150)  at com.atlassian.plugin.manager.DefaultPluginManager.earlyStartup(DefaultPluginManager.java:573)  at com.atlassian.jira.plugin.JiraPluginManager.earlyStartup(JiraPluginManager.java:118)  at com.atlassian.jira.ComponentManager$PluginSystem.earlyStartup(ComponentManager.java:610)  at com.atlassian.jira.ComponentManager.quickStart(ComponentManager.java:188)  at com.atlassian.jira.ComponentManager.start(ComponentManager.java:157)  at com.atlassian.jira.upgrade.PluginSystemLauncher.start(PluginSystemLauncher.java:43)  at com.atlassian.jira.startup.DefaultJiraLauncher.lambda$postDbLaunch$2(DefaultJiraLauncher.java:146)  at com.atlassian.jira.config.database.DatabaseConfigurationManagerImpl.doNowOrEnqueue(DatabaseConfigurationManagerImpl.java:298)  at com.atlassian.jira.config.database.DatabaseConfigurationManagerImpl.doNowOrWhenDatabaseActivated(DatabaseConfigurationManagerImpl.java:194)  at com.atlassian.jira.startup.DefaultJiraLauncher.postDbLaunch(DefaultJiraLauncher.java:137)  at com.atlassian.jira.startup.DefaultJiraLauncher.lambda$start$0(DefaultJiraLauncher.java:103)  at com.atlassian.jira.util.devspeed.JiraDevSpeedTimer.run(JiraDevSpeedTimer.java:31)  at com.atlassian.jira.startup.DefaultJiraLauncher.start(DefaultJiraLauncher.java:101)  at com.atlassian.jira.startup.LauncherContextListener.initSlowStuff(LauncherContextListener.java:115)  at java.lang.Thread.run(Thread.java:745)

[Sathish Kumar Venkataraman]

Related question.. The following plugin.. is this supported? JIRA update checker tells that

Unknown

These add-ons have not been registered with the Atlassian Marketplace, so their compatibility with JIRA 7.1.7 is unknown. We recommend performing your own testing before you proceed.

Atlassian JIRA - Plugins - Mail Plugin

If this above plugin is not supported, what is the alternative?  Also please see the exception that I am getting in the original report.