Is it possible to add SPF and DKIM records to validate transactional emails from Zoho Invoice?
Yes. You can include zohobooks.com to your SPF (Sender Policy Framework)record. This entry represents all mail sending services of Zoho, and therefore including this entry would be sufficient if you use any other Zoho Product as well.
We hope that adding SPF record would be enough and there would no need for adding our public DKIM keys.
However, feel free to contact support[at]zohoinvoice[dot]com should you require any assistance on this regard.