Brian, Outlook Express hasn't been supported by Microsoft since April 2014 when Windows XP support ended. I'm not even sure where you would look to find out what Symantec says about it.
As a managed services provider, I gave my clients 6 months of leeway before telling them that if a vendor no longer supports a product, then neither can my company. The liability is too great if something untoward happens.
DoubleGJ, I suggest rather than saying you can't buy this software, you try to convince your client to move to a platform that is supported, and move the OE app to either the cloud or desktop version of Outlook (both of which Symantec.cloud supports).
Microsoft Windows 7 Professional is still available from OEMs and distribution until October 31, 2016 - that's Microsoft's end-of-sale deadline. Come November 1, only Windows 10 can be sold.