Genuine Norton emails?

I have received an email from 'Norton' inviting me to get my free Norton Internet Security free update today. I am invited to click the box 'Update now'.  Is this real or pishing?  It supposedly comes from Norton@nortonfromsymantec.com but why is it offering me a free upgrade when my subscription is still in date and I am meant to get all updates automatically?  I haven't done anything with it as felt it looked a bit suspicious.  Any advice please?  I'm a bit green on these matters!  Many thanks.