(domain name below obscured for security reasons)

I am writing a new website which has a PHP form designed to post the data from an HTML5 form and send the data as an e-mail via Zoho's servers to our info@***.com e-mail account (which is hosted by Zoho). I should note that the MX records for our domain point to mx.zohomail.com and mx2.zohomail.com, and have been for some time now. I am using the following details in the PHP mailer:

SMTP Server: smtp.zoho.com

Port: 587

Account: info@***.com

Password: usual password used to log in to the account (verified working)

Recipient: info@***.com (same as sending account)

However, whenever the form is submitted, no e-mail is received at the info@***.com address.  The form seems to connect and submit OK as there is no delay when clicking the submit button. I have tried changing the recipient to my @outlook.com address and this doesn't receive the e-mail either.

I should note that the form is currently sat on a different web server to the one pointed to by www.***.com; I have a test server set up which I am trying to get this working on before the site goes live.  Unfortunately this test server does not have a logging capability so I am unable to see what the problem is on my end.

The PHP mailer is definitely working; I have used this on a number of other website projects with no issues, and in fact if I edit it to send via my own mail server's SMTP it works perfectly.  In this case however, I cannot permanently use my mail server so I need to send via Zoho's SMTP.

This must be an issue with Zoho's mail servers not accepting mail from the form for some reason.  Could it be that the IP addresses of www.***.com and my private test server are not authorised to send mail? If so, how do I go about authorising them, and can this be done within the Zoho Control Panel?

Any help you can offer would be greatly appreciated.