Email actually can prove a lot.
Logs are really useful for things like proof of delivery. If the recipient server responded with 250 for example, it means that the message was sent and accepted. Even if the server lied and deleted the message, you have proof that the message was sent and that whatever problems the recipient had were their own responsibility.
Use of sent email records (i.e., backup record of messages that have been sent, which is the topic) are very useful for forensics. Say a client complains about threats sent by one of your employees, if you have a record of all the messages sent through your server, you have what is needed to corroborate or deny their claims as well as a full record of what actually happened.
These are used all the time when dealing with governments. Here (Ontario, Canada), the criminal left-winged prior government was caught out on a lot of criminal activity through email records.
Then of course there’s this; https://www.documentcloud.org/documents/20793561-leopold-nih-foia-anthony-fauci-emails
Now to the actual question of the thread;
ABSOLUTELTY it can be done, but it has nothing to do with Nextcloud. What you need to do is set up the
always_bcc parameter or
sender_bcc_maps in Postfix (the SMTP server). The full details of that are, of course, well beyond the scope of this forum, but that should be enough information for you to start off with.