Over on the AOL Postmaster blog, Lili Crowley announced yesterday that AOL has made changes to their spam filtering system. Specifically, more senders may be subject to blocking with CON:B1 errors. AOL’s website explains that CON:B1 errors indicate that an IP address is being blocked “due to a spike in unfavorable e-mail statistics.” This strongly suggests that a sender blocked with a CON:B1 error message has a negative sending reputation. This is yet another data point as to how ISPs have been tightening up spam filtering and reputation requirements over the past few years. What you might have been able to deliver five years ago, you might not be able to get delivered today.