Mach 4 Network on SPF/DKIM/DMARC -by scc April 2018 upd: May 2019

Sender Policy Framework (SPF) — email-validation system designed to help detect email spoofing: provides a mechanism for recipient to determine if an incoming mail is from a an authorized host for the sending domain.
merely enables the recipient, should he/she so wish, to authenticate sender according to DNS records
Mach 4 Notes
SMB: SOHO 1 or 2 users | very small 3-10 | small 10-24 |  medium 25-50 | large 50-100