Hi Stephane,
You wrote that after enabling SMTP node all faxes was routed to them, so i assume that SAPconnect is using SMTP node as first and FAX as a second node.
Try to filter messages on first SMTP node and (i think) rest of them will be sent via FAX (2nd) node. It's only my supposition but maybe it will work.
I think that more secure and stable resolution will be setting ranges of addreses for suitable nodes. Using wildcard '*' in this case is risky.
Regards
MKrzywko