Headers is a read-only TIdHeaderList property used to represent the RFC message headers for the Internet Message in NAME=VALUE form. Headers represents the header values as described in the Internet Standards documents: 

RFC 2045 - Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies  

RFC 2046 - Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types  

RFC 2047 - Multipurpose Internet Mail Extensions (MIME) Part Three: Message Header Extensions for Non-ASCII Text  

Use properties and methods in the message part to maintain values stored in the Headers property. 

Use ExtraHeaders for custom or experimental message headers not represented as properties in the message part.

TIdHeaderList 

TIdMessage 

TIdMessagePart.Headers 

TIdMessagePart.ExtraHeaders