Ask HN: Is there a good multi provider multi accounts email archiving solution?

I have several work email accounts, used by several users, and I would like to put in place better archiving and backup than what there is in place already.

Looking around, what I seem to want would be similar to Gmail Vault, so that I have a trace of all emails going through while still letting each user be the master of his own inbox, but the cost of passing all the email box we use on g-suite with the added cost of the vault option make it too expensive at the moment.

I'm looking for a similar self hosted solution, maybe some kind of imap/pop/smtp proxy that would grab email from external providers, and then users would connect to it to retrieve their mail or send new mails ?

Ideally, I would like the resulting email to be stored in an accessible way so that I can also process them to run some automation.