Identifying which major Mailbox Providers (MBPs) use Cloudmark for spam filtering is complex due to a combination of factors: Cloudmark's confidentiality policies, the integration of Cloudmark with other filtering systems, the use of reseller agreements by smaller ISPs, and the dynamic nature of spam filtering solutions. While Comcast is a known user, along with indications of usage by Charter, Cox, and Roadrunner, many providers combine Cloudmark with in-house or other third-party solutions like Vade or Synacor. Proofpoint, Cloudmark's parent company, offers wider email security solutions used by various MBPs. Analyzing email headers for 'X-Cloudmark-Analysis' or monitoring reputation for Cloudmark blocks can provide clues. Ultimately, focusing on strong email authentication (SPF, DKIM, DMARC) and deliverability best practices is crucial, regardless of the specific spam filter in use.