Knowledge base: putting free proxies to work, by the numbers

Each entry takes one job, counts the addresses it needs, names the ways a free proxy fails at it and says when a paid pool is the cheaper option.

• Checking an email list through proxies: SMTP verification without burning your IP
  Why one address gets greylisted after a few hundred RCPT probes, how many IPs a 100,000-email list needs, and why a public proxy fails an SMTP check.