Hi,
We all know that only OOB fields have been restricted from Field Level Security and all the custom fields can have Field Level Security. However, today i tried creating a new field of type “Single Line of Text” of format”EMail” with “Field Level Security” enabled.To my surprise I can see the following error:

And this is the error message in the Log file: “This field is not
securable”

I raised this issue with MSConnect here . Need to findout whether its bug or by design.