Until 5.3, the envelope sender was set to a bounce address of the domain of the openacs instance. Unfortunately that has been lost during the refactoring of acs-mail-lite for 5.4.
I just committed a patch to both HEAD and oacs-5-4 to set the envelope sender using the bounce address, as it used to be, and to pass it to smtp::sendmessage using the -originator switch. That should fix your problem.