What's the difference between mx: and include: in SPF records?
"include:" includes the settings from another SPF-record (must point to a different name) - it does not actually specify any hosts ifself.
"mx:" includes the hosts designated by MX-records for a domain name (same a using "a:" for each of the host names pointed to by the MX-records).
The 'include' directive allows you to import SPF settings from another zone. Imagine if you are a company that has 5-7 zone. Rather than writing the SPF records each time, you can just write them on one and import to the others.
Hope that helps!