virus protection exchange server

You don't say what version of Exchange.
(Microsoft used to have a good product called Forefont but they don't sell it any more.)

You can run a separate SMTP filter (like X-Wall).
You can also use a firewall with an SMTP filter addin like SonicWALL that does some of the work before the message gets to your Exchange server
You can buy a black box like Barracuda
You can outsource the preprocessing to Microsoft or McAfee (after all most of your e-mail comes from the Internet.
You should also have the mail filtering turned on on the client endpoints.

Exchange 2013

I have Symantec Endpoint for Small Business.  In the cloud version, does it provide mail filtering?

So no Forefront.

But all the other suggestions are valid, You should have endpoint protection in the Outlook / web clients and something between your server and the Internet.

http://www.symantec.com/email/

The second section is on on-premise protection

But if you have Symantec on the workstations it might be better to have a different provider elsewhere.

Does Cisco iron port provide such a function?

Microsoft:
https://products.office.com/en-us/exchange/microsoft-exchange-online-protection-email-filter-and-anti-spam-protection-email-security-email-spam

McAfee
http://www.mcafee.com/us/products/saas-email-protection-and-continuity.aspx

But there are plenty more.

Yes Iron Port does that too.

We use Mimecast as our email anti-spam AV protection.  It's a cloud-based service so it take care of emails before they hit you and it works pretty well. We have been on it for over two years and it has done a good job for us.