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If you have added or updated your FuseIT license key within the S4SLicense.xml file and it is still not being recognised by Sitecore you may need to reset your IIS Application pool and the IIS Sitecore web application. If you have an Expired License message you will need to obtain a new S4S Evaluation version or license from FuseIT.
FuseIT SFDC Explorer
This is a free developer tool that is an application you can install in a Windows environment. Internally it uses the S4S engine to connect with Salesforce and is ideal to verify your connection credentials to Salesforce.
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