An Intermediate Service Provider (ISP) is an entity that facilitates the transfer of information between two parties. In the context of Form 8821, Tax Information Authorization, an ISP is a person or entity that is authorized to receive tax information from the IRS on behalf of the taxpayer. The taxpayer completes Form 8821 and designates the ISP as the person or entity to receive the tax information. Form 8821 is used when the taxpayer wants to authorize someone other than themselves to receive their tax information from the IRS.