When trying to validate your email host to use with ZOHO books, the advice given by zoho (for us) was rather misleading.. It asks you to add a line of code into the DNS settings of your Email hosting service (SPF).  What transpires in our case after several days of ZOHO attempting to solve the issue is, you are only allow ONE line of SPF code in the DNS settings and, in our case, this was already there.  (you can use a free service online called MXtools to check)  SO.. I put a call into the support team of our email host, they said they could easily sort it.  I sent the line of code ZOHO books recommended to use and they INCORPORATED into the single SPF line of code that was already there.  in the space of 10 minutes the validate button worked and we were away..