The sys_email table uses a Document ID field(s) to relate back to records, not a direct relationship. So you will need to create a user defined related list:
Creating Defined Related Lists - ServiceNow Wiki
1. Go to System Definition\Relationships and click New
2. Name what you want to call this related list in the Name field- example "Email Log"
3. In applies to table choose incident - or if you want this to all tasks you can choose task
4. In queries from table choose Email
5. In the query with script field, paste in:
(function refineQuery(current, parent) {
current.addQuery("target_table", parent.sys_class_name);
current.addQuery("instance", parent.sys_id);
})(current, parent);
6. Click Submit.
7. Go to your incident form, configure related lists, and add "Email Log" or whatever you called the relationship in step 2 above.
