Roger
All (I think) email servers produce data on an email received which can be viewed by the sender if they select that option. They typically also show some or all of the intermediate router information.
This information is normally held in a log on the receiving mail server. You can configure how long the data is retained for if you have control of that server.
If that data is still available depends on how your server is configured. For example on my Business email server (which I effectively rent) I can access all the logs and they are configured as "loop storage" for logs, so how long the data is retained depends on the number of email I get.
I could pay more and get more storage.
If you are using a large email supplier (BT, Google etc) then I suspect the logs for each email are not kept very long and you would probably need a court order to see them (none of that would apply to GCHQ, as they seem to get all the "Meta Data").
Back to your question, My view is that you can easily produce a fake "email delivery report" even showing the recipient's IP address. If the sender's email provider can produce a report then most courts would accept that, if the report only came from the sender then it is not a proof but a court might accept it (as a balance of probability) unless you have some evidence to suggest otherwise.
I have experienced cases where a Post Office "proof of postage" was taken by the Judge as evidence of delivery!
If the sender is saying they have a "delivered" status only and not a "delivered and read" status then that could mean the email was delivered but never read (or went to spam). But not having an email "delivered and read" status does not mean the email was not read, only that the recipient may have decided not to acknowledge it was read.
