How do blocklists differ in severity or reach?

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.