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.
===
A quick of google led me to
RFC 6758 - Tunneling of SMTP Message Transfer Priorities
A key phrase - why I do not expect to find any support in AIX, is this:
Quote:
A number of other header fields are already in use, mostly in Message User Agents (MUAs), to convey meanings related to importance or priority of messages. Examples of such header fields are Importance [
RFC2156], Priority [
RFC2156],
and X-Priority (undocumented).
Hope this helps!