From the 14th July 2015 Microsoft Small Business Server 2003 is no longer supported. In practical terms, this means no more support, no more patches and no more security updates.
To put this in perspective, there were 37 updates in 2013 alone. With no more updates and patches, continuing to run a 2003 Server environment will mean a less stable and less secure network. Payment systems such as Paypal, Visa and Mastercard will no longer support Sever 2003 from the end of support date.