How do blocklists differ in severity or reach?

Blocklist severity comparison:

High impact (widely used):

Spamhaus (SBL, XBL, DBL) - used by most major providers

Barracuda - common in corporate environments

Proofpoint/Cloudmark - enterprise filters

Medium impact:

Spamcop - automatic, transient, report-driven

SURBL - URI checking

Various corporate security vendors

Lower impact:

Smaller regional lists

Niche or specialized lists

Lists with limited adoption

Prioritization:

Focus on Spamhaus first (greatest impact)

Address Barracuda for B2B

Research which lists your target audience's providers use

Some port authorities are more influential than others. Prioritize the ones that matter most.