Zoho email and Digital Ocean

I have set up a Zoho email account to handle my email delivery from my site on Digitial Ocean.

I have an MX record for the site pointing to the Zoho server, and the emails are being delivered, but sometimes flagged as spam, depending on how sensitive the receiving email client is. My Yahoo account receives them fine; my Gmail account receives them but with a warning that sender can’t be confirmed. A third account sends them to spam.

The Zoho help files mentions the following, which may be the issue –

“In case you use webforms in your websites, which sends emails, the email sending server at your web-host, in most cases, tries to deliver the email locally, as the domain name in the ‘To Address’ matches with the domain name used with the website. Hence make sure that you configure ‘Remote Server’ for email delivery to ensure that you receive emails without any errors. Switch off any Local Host or Local email delivery configuration, which causes the emails to not reach the Zoho Mail servers at all.”

Does anyone have any ideas on how to set up my server to switch off local email delivery, or is this perhaps not the issue?

Thanks