1. Finance
2. Medicine
3. Aeronautics
There are likely more I'm personally unaware of. In finance, which I'm most familiar with, lenders (sometimes banks, but can be things like credit unions, marketplace lenders, or investors) have a laundry list of regulations they need to meet in order to business in the US. Many of these regulations explicitly require that the lender produce any and all communications between the lender and borrower on-demand--and for good reason.
There's a long history of bad actors in finance lying or misrepresenting things in official communications (fraud, embezzlement, money laundering, etc). If you work for any finance company, you had better expect that everything you do on your corp accounts/devices will be logged, audited, and periodically reviewed.
That said, the only time I've heard of an employees' email being pulled out of an archive and read were due to concerns around IP theft or questionable behavior between said employee and business clients.
This is why traders WFH during coronavirus lockdowns is so problematic; not only a lack of externally accessible recorded phone lines but the impossibility of policing OOB communications when the local compliance guy can’t physically see you talking on your mobile phone.
What really concerns me is that these phishing attempts are just the attacks we're observing. Many more will go unnoticed given how ad-hoc current work environments have become with so many organizations going full-time remote overnight without the IT/security systems and processes in place to safely support it.
Not if phones are checked in at the security desk when you arrive, there is a phone jammer operating in the building, and compliance staff are patrolling to make sure no one has smuggled a phone in. These measures are normal on most trading floors.
Because I work in aeronautics, and we've never had such requirement. Unless my company retains all deleted emails without telling us.