Test cases that are not so obvious

In my current assignment I need to identify test cases/scenarios that are out of the ordinary. I am testing a professional networking site liked linkedIN but one that is targeted to a specific community. It has features such as profile building, connections, watches, messaging, Network updates, Groups, career news, job posting etc that a social networking/professional networking site will have. We already have about 1500 test cases covering the major functionality of the application. But the cases were written by inexperienced testers and cover only the obvious stuff. Today my boss told me to create cases that are not very obvious and only an experienced hand would be able to point out. He pointed scenarios such as
1)When SMTP server is down
2)SPAM filter filters out the invitation mail sent by the system
3)What happens if the user clicks the invitation link after its validity runs out
4)browser session expiry

I have to create cases for these and many other scenarios that are not so obvious. I have been working in IT for a while but  my experience in identifying/creating test cases is limited. My company however expects me to perform like an expert tester.
These cases i would imagine are common to all applications and are not specific to any one domain or technology.
I need the following
a) Please share with me any such scenarios.
b) Is there a book i can refer to for such scenarios or is there any resource available online that points out such cases.
Thanks for reading