Mail Security Tools is a directory of about 99 email security and MSP tools in 11 categories, with 8 guides on spoofing, authentication and common gaps. It also offers a free DNS check covering MX, SPF, DMARC and DKIM records, and the check does not keep your domain.
A $29 PDF email security checklist is sold as a one-time purchase, and all sales are final. The site is run by DME Computer Services in Omaha, and it discloses its own tools and sponsored slots.
Mail Security Tools exists because email authentication gaps keep causing avoidable problems, and a clear set of guides, tools and a checklist helps small teams close them.