Emails generated by my website not now accepted by some servers after transferring to Zoho

I have transferred all my email accounts for ehnetwork.org.uk to zoho.
In most cases that has worked fine.

However, my website still sends out emails and needs to continue to do so- notifications of new content, occasional newsletters, and admin emails like password resets. The website email domain matches the domain ehnetwork - currently site@ehnetwork.org.uk.

A small minority of servers seem to be rejecting those website-generated emails because the domain is not recognised. I get things like:

"Domain of sender address site@ehnetwork.org.uk does not exist"

5.1.0 - Unknown address error 553-'5.1.8 <site@ehnetwork.org.uk>... Domain of sender address site@ehnetwork.org.uk does not exist'

Received-SPF: Fail (EXET1.barking-dagenham.gov.uk: domain of site@ehnetwork.org.uk 
does not designate 195.245.231.135 as permitted sender) receiver=EXET1.barking-dagenham.gov.uk; client-ip=195.245.231.135;helo=mail6.bemta5.messagelabs.com;

I have already changed the website email address from admin@ehnetwork to site@ehnetwork (as admin@ was hosted by zoho and I thought this might create a direct conflict). But that doesn't seem to have helped - probably because the mx record only refers to the domain anyway.

Is there a setting within Zoho that will overcome this? I see something in control panel around Allowed IPs. Or if not, what is suggested.