Need to custom identify incoming outlook messages and keep thisidentifier when user replies

[Thore Berntsen]

I need to map an incoming message with the replying outgoing messages
using a custom identifier. The incoming message should be built
including this custom identifier and sent to the end user in outlook.
We could add this identifier as part of the e-mail subject, but prefer
to add it somewhere that the end user cannot edit.
When the end user replies to the message, it is important that this
custom identifier is included in the reply message, just as it would
if we included it in the subject field.

Is there a document field that can be used for this purpose, and will
the value in this field be transferred into the reply-message?
If so, will it work for every recepient without having to make any
changes, or must it be published for every recepient separately?