I am hosting an web application for many domains. The web app does send emails on their behalf. Right now, I am requiring my clients to add me onto their SPF record like a:smtp.masterdomain.com, but realized the "include" directive and need help making it work. The reason why I want to use the "include" directive is because I am adding a second smtp server that will also be sending emails for the web app, such as smtp2.masterdomain.com. I do not want to continue to ask my clients to add an smtp server every time I add one.
Say my application is hosted at masterdomain.com and is sending emails on behalf of firstname.lastname@example.org, email@example.com, and firstname.lastname@example.org. It is being sent by my mail servers smtp.masterdomain.com and smtp2.masterdomain.com.
Can I tell my clients to add an SPF record like: v=spf1 include:spf.masterdomain.com -all so I can manage the SPF records on their behalf? What would I put in the spf.masterdomain.com spf record to make the SPF pass for smtp.masterdomain.com and smtp2.masterdomain.com for sending emails for email@example.com, firstname.lastname@example.org, and email@example.com?