If your website is hosted with Synergy Wholesale, add the bold text below to your spf record.
v=spf1 a include:spf.hostyourservices.net ~all
It includes all IP ranges across all synergy platforms.
You can also do tests email sends from the website, and check the email headers, if you see an ipv6 address, you may use that (ip6:<ip6 address> instead of include:spf.hostyourservices.net) because that ipv6 address does not change, as that IP address is tied to the server you're on. It would only change if you moved to a different server.
Credit to Owen Barrett at Technical Support Representative, Synergy Wholesale.