Zoho Books API invoice email bouncing with 'relaying-issues' error

I have waited over 30 days for zoho books uk to assist with the following and i have had no replies or tickets erronously closed. The service has been terrible - very unlike zoho!

So i am raising this here hoping that a community member can assist:

Hello,

I'm using the Zoho Books API to send invoices via email, but receiving bounce notifications with "relaying-issues" error.

Setup:
- Domain: businessoverheadsolutions.co.uk
- Sender: commissions@businessoverheadsolutions.co.uk (DKIM configured and verified)
- Recipient: I&Ce mail@abc.com (have removed main part of email for anon)
- API endpoint: POST /invoices/{invoice_id}/email
- Payload: {"to_mail_ids": ["I&Ce mail@abc.com"], "send_from_org_email_id": true}

Bounce Details:
- Bounced Address: I&Ce mail@abc.com
- Bounce Reason: relaying-issues
- Invoice: INV-001789

Questions:
1. Does the API email endpoint have different relay restrictions than manual sends?
2. Are there additional DNS/authentication records needed for API-sent emails?
3. Is there a whitelist or verification process for external recipient domains?

I've verified DKIM is configured. Testing manual send now to compare results.

Please advise on proper configuration for API-based invoice email delivery.

UPDATE: Manual sends from Zoho Books also fail with the same relay-issues error to this recipient. However, sending the same invoice PDF manually via Outlook/Zoho Mail (using the same authenticated domain) delivers successfully.

This suggests the issue is specifically with Zoho Books' mail servers being blocked by the recipient's email provider, not our domain authentication.