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, 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 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 and is sending emails on behalf of,, and It is being sent by my mail servers and

Can I tell my clients to add an SPF record like: v=spf1 -all so I can manage the SPF records on their behalf? What would I put in the spf record to make the SPF pass for and for sending emails for,, and
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create the SPF record for 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, and to add the to their SPF records. That should be all you have to do.

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