function DecodeHeader(
    Header: string
): string;

String - Value for the decoded messsage header.

DecodeHeader is a String function that decodes the RFC 2822-compliant message header, specified in Header, that has content transfer or character set encodings. 

DecodeHeader checks Header for an ISO or WINDOWS character set encoding marker ("=?ISO" or "=?WINDOWS") to determine when character set encoding has been applied to Header. A specific check is performed for the ISO-2022-JP character set encoding. 

DecodeHeader also looks for the content transfer encoding markers for Base64 ("?B") or Quoted-Printable ("?Q") encoding. DecodeHeader then decodes all encoded characters until the final "?=" marker is located. 

DecodeHeader will remove any Null strings (Decimal 0) embedded in the message header using Base64 encoding.