EXOEnableArchiving.ps1

<# This script enables archiving for users who have reached > <setamount> on their mailbox, in order to clear up space and maintain retention policy.
you simply need to set your threshold for archiving to kick off, the name of the retention policy you want to use, and organzation domain.#>

Import-Module ExchangeOnlineManagement
$rp = '' # Retention Policy Name
$mbthreshold = '' # Mailbox threshold
$orgDomain = '' # Organization Domain

# Connect to EXO with managed identity
Connect-ExchangeOnline -ManagedIdentity -Organization $orgDomain

# Retrieve all mailboxes and gather current mailbox size
$mbabove89 = ForEach ($mailbox in (Get-EXOMailbox)) {
    $stats = $mailbox | Get-EXOMailboxStatistics | Where-Object {$_.TotalItemSize.Value.ToGB() -gt $mbthreshold}
    New-Object -TypeName PSObject -Property @{
        'Name' = $mailbox.UserPrincipalName
        'TotalItemSize' = $stats.TotalItemSize
    }
}

# Merging Properties into one table
$mbtoenablearchive = $mbabove89 | Select-Object Name, @{Name="TotalItemSize";Expression={$_.TotalItemSize.Value.ToGB()}} | Where-Object { $_.TotalItemSize -gt $mbthreshold } | Sort-Object -Property TotalItemSize -Descending

# Enabling archive and setting retention policy
ForEach ($mb in $mbtoenablearchive) {
    Enable-Mailbox -Identity $mb.Name -Archive
    Set-Mailbox -Identity $mb.Name -RetentionPolicy $rp
}

# Writing to screen for logging purposes
$mbtoenablearchive | Write-Output