This can happen because SPFBL has 3 different response codes for their blacklist:

  • – which means that your IP is blacklisted for confirmed spam
  • – which means that your IP is only suspected of sending spam, or it’s misconfigured (not in compliance with RFC 5321)
  • – which means that your IP isn’t running an email server, or it’s a NAT router, or it’s a residential connection

You can read more about their response codes here:

Our platform only considers an IP as being blacklisted on this RBL for the first response code.

We have done this in order to prevent a lot of false positives.

So, if your IP is misonfigured, or a NAT, or a home connection, or only suspected of spam, but not actually confirmed of sending spam, this RBL will return the second or third response code, which we do not consider as being blacklisted, but other blacklist monitoring services may show as being blacklisted, and this is where the discrepancy in results may come from.