In /etc/MailScanner/spamassassin.conf, this is set by default.
envelope_sender_header X-org-name-MailScanner-From
It seems that setting it in a local configuration file located in /etc/spamassassin/ do not work.
A tried with zzz_local.cf to be sure it will be read after Mailscanner.conf.
If I comment the envelope_sender_header in /etc/MailScanner/spamassassin.conf, the one in local.cf works.
Bizarre.