If you have a FEEDBACK LOOP setup with Google, and they receive regular, albeit erroneous complaints about your domain, or a specific domain you host. they will STRONGLY remind you that SPF is now "required."
We take that attitude of REQUIRING SPD, DKIM, DOMAIN KEYS, and preferring DMARC as well - and vet spam very highly on all three of them
If we are gong to control spam, then we all need to get in the same boat and effect all available tools - nothing external, it's all built into SmarterMail, but does require both an excellent knowledge of both, and the use of a PRIVATE DNS server, with at least one secondary DNS server on a different network.
Phonr: (773) 491-9019
Phone: (224) 444-0169
E-Mail and DNS Security Specialist
Network Security Specialist
Customer Service Portal: https://portal.chicagonettech.com
Security Blog: http://networkbastion.blogspot.com/
Web and E-Mail Hosting, E-Mail Security and Consulting