If you've got user metadata, you can search specific fields by using a fieldname like u_yourfield. So if you've got a metadata field being indexed called "userid", you could search that by using a query such as this:
u_userid:111*
To search by sender address, you can use "sender" as the field name; to search by recipient, use "full_email" as the field name; Subject line is "subject". If you want to search for people who clicked on a specific URL, you can use something like url:twitter.com* to get results for people who clicked on URLs that start with twitter.com.
--Kaitlin