Hi,
pass in the case guid into "passinthecaseid", refer below link on how to construct the query using fetchxml
[View:https://msdn.microsoft.com/en-us/library/gg328117.aspx:750:50]
Sample code ( please read through the above link)
<fetch version='1.0' output-format='xml-platform' mapping='logical' distinct='false'>
<entity name='email'>
<attribute name='subject' />
<attribute name='regardingobjectid' />
<attribute name='from' />
<attribute name='to' />
<attribute name='prioritycode' />
<attribute name='statuscode' />
<attribute name='modifiedon' />
<attribute name='activityid' />
<attribute name='description' />
<order attribute='subject' descending='false' />
<filter type='and'>
<condition attribute='regardingobjectid' operator='eq' value = '"+passinthecaseid+"' />
</filter>
</entity>
</fetch>