Actually

-> RCPT TO:<[log in to unmask]>
553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
-> No recipient left, message aborted.

that 553 error is coming from qmail, not LISTSERV.  LISTSERV does indeed
operate by opening port 25 and sending SMTP mail through the local MTA,
so if you have relaying turned off in qmail I'd imagine you'd see this
error.  Since LISTSERV isn't being allowed to relay, it aborts the
message.  GAEA would have to be set up to allow relay from the local
host if not anywhere else.

I think you'd have better luck if you asked this question on LSTSRV-L
but possibly someone here will know more about the LISTSERV/qmail
interaction than I do.

Nathan