Here's a link to a detailed Zoho article on how to ensure better email deliverability:

Video Overview on Managing SPF Settings: https://workdrive.zohoexternal.com/external/6Oj9WsXdtDb-Jd0rW

If send marketing email fro Zoho CRM, with your domain email connected to Zoho CRM, here are some instructions for setting up SPF verification:

Here's a Google article explaining SPF verification for GSuite mail:

Here's a Microsoft article explaining SPF verification for Office365 mail:

Here's a Google article explaining SPF verification for Zoho hosted mail:

If your email is hosted elsewhere, check with your email host for further instructions.