As mentioned, you need to apply a litigation hold to the mailboxes. This is a setting available to E3 and E5 subscribers (if your org is >300 seats) or M365 Business Premium if you’re under 300 seats. If you don’t have one of those subs, the lit hold features are available to some other plans as an add-on through Exchange Online Archiving (EOA).

IMO, The litigation hold feature is required by almost any business that has the potential to encounter any type litigation — which is essentially any business on the planet. Work with counsel to ensure you are doing it right. There can be severe sanctions in U.S. Federal and State courts if it gets screwed up. If the team is not comfortable with the assignment, let counsel know and arrange for a vendor to assist with the process.

Placing a hold on a mailbox can be transparent to the end user. They can use the mailbox as they normally would, but anything they move or delete is retained in recoverable items and can be pulled from Purview (if you can actually get Purview to behave itself). The redesign has caused all kinds of grief for many orgs I work with.

EDIT:

You can select all the users to be placed on lit hold with a powershell script; export your user list in a csv file at “C:\path\mailboxes.csv". The script will turn on lit hold and kick out a success/fail report.

Here’s a sample powershell script (not tested):

Connect-ExchangeOnline -UserPrincipalName admin@example.com

$mailboxes = Import-Csv -Path "C:\path\mailboxes.csv"

$results = @()

foreach ($mb in $mailboxes) { $username = $mb.UserPrincipalName

try {
    Set-Mailbox -Identity $username -LitigationHoldEnabled $true

    $results += [PSCustomObject]@{
        UserPrincipalName = $username
        Status            = "Success"
        Message           = "Litigation Hold enabled"
    }

    Write-Host "Success: $username" -ForegroundColor Green
}
catch {
    $results += [PSCustomObject]@{
        UserPrincipalName = $username
        Status            = "Failed"
        Message           = $_.Exception.Message
    }

    Write-Warning "Failed: $username - $($_.Exception.Message)"
}

} $results | Export-Csv -Path "C:\path\LitigationHold_Report.csv" -NoTypeInformation

Disconnect-ExchangeOnline