How to centralize SPF records to one domain for multiple SMTP servers?

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 info@domain1.com, info@domain2.com, and info@domain3.com. 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 info@domain1.com, info@domain2.com, and info@domain3.com?
bemara57Asked:
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jar3817Commented:
create the SPF record for spf.masterdomain.com and in it, just list the 2 IPv4 addresses of your smtp servers. Don't list them as dns names, that'll just require more dns lookups.

Then tell the admins of domain1.com, domain2.com and domain3.com to add the include:spf.masterdomain.com to their SPF records. That should be all you have to do.
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