That may be a special feature in HP. When I see an email header
X-something: Some Value
I try to remember that for the SMTP protocol it is probably not known. There are many RFC's that govern mail - and if this is one that is specified in an RFC - so it is part of an accepted, especially mandatory standard, and the supported versions of AIX are not supporting it - submit a PMR.
If it is not a standard, submit a a request for a design change.
And, as it is about SMTP/mail handling - a query in a more "mail" oriented forum may get you better results. - Even ask on HP, as it is a known feature there.