Invoices not arriving and mail server settings

Invoices not arriving and mail server settings

I am having an issue where some clients are not receiving invoices. I have configured Zoho Books to send on my behalf and configured the appropriate SPF, DKIM and DMARC settings on my mail server and tested these as working.

I get the CC'd copies so I know they are being sent. I suspect the clients mail filters are blocking the 'eu-sender.zohobooks.com' domain.

I have tried added my work Google account as a 'mail relay' and I received the confirmation from 'Zoho Team' that this was working. However when I have tested automatic invoices since adding this they still arrive as 'mailed-by: eu-sender.zohobooks.com'

With a 'mail relay' setup will all mail including automatic invoices be sent via the mail relay or just manual messages? Will it still say 'mailed-by: eu-sender.zohobooks.com' because it originated from Zoho even though it's going via the relay?

I've also tried to white list the sender in the mail filter but it's a number of clients with the issue so it's proving quite difficult.