An SPF record is a TXT entry in your domain’s DNS specifying what servers are validated to send mail for your domain.
To specify Greatmail as the only service provider authorized to send emails for your domain, you would use the following syntax:
v=spf1 mx include:spf.greatmail.com -all
To include another service provider in the SPF record, for example, IP address 64.233.167.97, you would add:
ip4:64.233.167.97
The final record would read:
v=spf1 mx include:spf.greatmail.com ip4:64.233.167.97 -all