The user can then enter: Disposition-Notification-To: "Me Yu" <..hidden..> X-MSMail-Priority: High as he or she wishes.Hmmm... that might be error prone... One wonders about MTA's and MUA's handling email with mysspelled [sic] ;-) headers....
An MTA/MUA will ignore all headers it doesn't understand. However, I do not think this is a feature that should be involved in LedgerSMB outside of standard headers that MUAs support.
A user should be able to use a check box to determine if they want the header sent with the email but arbitrary headers should not be allowed. That would just be a foot gun.
... My own thinking is that some things ought to be removed (like ship via) since they are probably better covered in AR/Transactions. Other things like Salesperson...This doesn't affect me, but one might imagine a salesman wishing to give his customers a call for a quick chat about their bills should his commission depend upon it. ;)
This seems to be leaning more in the CRM direction which LedgerSMB is currently not. There are plans to implement some CRM functionality into LedgerSMB for 1.3.
Sincerely, Joshua D. Drake