ReadHeader is a virtual procedure in
TIdMessageDecoder that specifies the method used to read an RFC-822 compliant message body.
TIdMessageDecoder descendants, like
TIdMessageDecoderMIME,
TIdMessageDecoderUUE, and TIdMessageDecoderXXE, must override ReadHeader to implement storage of parsed header names and values in
Headers. ReadHeader must also update
PartType based on header values spoecific to the message decoder class. Additional properties in the descendant may also require updates based detected Header values.