If you are behind a dynamic IP address, spamhaus is not going to remove that address from the PBL, since that is more or less the purpose of that flavor of blacklist.
I still believe the best solution would be to ask your ISP if they have an outbound mail server for their customers to use, and then configuring that as the mail relay server in the AMP setup page per my previous message. (If *that* outbound mailserver requires client auth, then you may be out of luck as far as Airwave messages go.)
Upgrading postfix will not, by itself, solve any client auth issues, because Airwave currently does not produce postfix config files that use outbound client auth.
EDIT: let me amend that slightly -- when I wrote the above paragraph I was thinking we overwrote main.cf every time you save the AMP Setup/General page, but in fact we use postconf to modify it. So it is possible you could hand-edit main.cf to use outbound client auth and Airwave would not disturb those settings in the course of normal operation.