Sabre misreporting IP address on blacklist check

Following a succession of spam registrations on my WordPress blog, i.e. meaningless username and no first name / second name details and no accompanying comments, I’ve added Sabre to my plugins and activated ‘Check DNS Blacklists’ on it’s ‘stealth’ options.

When I logged off of admin and tested registration with a set of dummy details, I received ‘IP address banned’ warnings and Sabre failed to proceed the registration.

I checked my IP(v4) on Spamhaus and SORBS but found no black listing.

I logged back in to admin and checked Sabre and it showed my failed registration under a different IP to that of my machine’s IPv4. Googling the IP led to a central london postcode.

Spamhaus reveals the following data about the IP that Sabre says is blacklisted:

86.148.172.83 is listed in the PBL, in the following records:
PBL231589

Ref: PBL231589
86.128.0.0/11 is listed on the Policy Block List (PBL)

It recommends that PBL is not a blacklist, and that turning on SMTP Authentication may allow things to proceed.

My questions are:
1) Is Sabre checking my IP, and if not which IP is it using?
2) Is it worthwhile using Sabre at all if ‘Check DNS Blacklists’ is deselected?
3) Does Sabre use SMTP authentication? (There is no option in the configuration panel).
4) and finally, what can I use to successfully rebut spam registrations if Sabre’s reliability is in question?

Any help is much appreciated.