I've filed this as a support request, however Oswaldo Hernandez has asserted that this is a bug and should be filed as a bug report, here:
https://jira.atlassian.com/browse/JRA-30451?focusedCommentId=576712&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-576712

I'm working on porting a custom email handler that was originally written for JIRA 5.1.6 to JIRA 6.1.7. The plugin compiles just fine, however it crashes at runtime with the following message:

[INFO] [talledLocalContainer] org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'com.omit.omit.EditHandlerDetailsWebAction': Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [com.omit.omit.EditHandlerDetailsWebAction]: Constructor threw exception; nested exception is java.lang.NoClassDefFoundError: com/atlassian/jira/plugins/mail/model/HandlerDetailsModel

Apparently it's not currently possible to include this class in JIRA 6.x.

Details here as well: https://answers.atlassian.com/questions/265936/plugin-porting-issue-noclassdeffounderror-com-atlassian-jira-plugins-mail-model-handlerdetailsmodel