How can I stop my Zendesk emails from going into my customer's spam folder?
In many cases, your email is directed to your customers spam folder because the customer's email system doesn't trust the sender. To improve the chances of a successful delivery, use a combination of SPF, DKIM, and DMARC to verify the trustability of your outgoing emails. When you set up SPF and DKIM, include Zendesk within the records to authorize sending from your account.
If issues persist, consider asking your end users to contact their mail providers to find out specifically why their system marked it as spam.
You can also try to correct the problem by adding these customers to your whitelist. For information see Using the whitelist and blacklist to control access to Zendesk Support.