[exim] OT email tags

Hi list

My deepest apologies for asking this here, but very desperate, and I
thought what more better place to ask an email question than in a MTA
mailing list.

I have to have perl create and send an email for my app. In the email
structure I need to have a tag to hold a reference number.

so my question is, can I just insert a tag in the mail headers ( I
assume is must be before the subject), or is any rules that need to be
followed before doing so.

I basically want my mail structure to be as so.

===========================================

from: abc@???
To: xyz@???
X-TAG: 1234567890
Subject: This is a mail structure I hoping will work.

This is the body of the message.

===========================================

I obviously dont want to X-TAG to be seen by the recipent, only in the
raw email viewing can it be seen.

If anyone could assist by pointing me to the some site / docs etc, it
would be most appreciated.
If anyone want to mail me, please mail me off the list (Think this will
accomodate the guys on the list from having to receive this thread).

Kind Regards and thanks inadvance
Brent Clark