# Locate all users that have not logged in within 12 months
$InactiveMonths = 12
$ExchangePowerShellEndpoint = "https://*Server*/PowerShell/"
$ExchangeAuthenticationModel = "Kerberos"
$UserCredential = Get-Credential -Message "Enter your credentials used to log into the Exchange Admin Portal"

$Months = (Get-Date).Addmonths(-($InactiveMonths))
$Users = Get-ADUser -Filter {LastLogonTimeStamp -lt $Months -and enabled -eq $true} -Properties LastLogonTimeStamp, mail |
select-object Name,@{Name="LastLogon"; Expression={[DateTime]::FromFileTime($_.lastLogonTimestamp).ToString('dd-MM-yyyy')}},SamAccountName,UserPrincipalName, mail

Write-Host "Located $($Users.count) users"

# Connect to Exchange
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri $ExchangePowerShellEndpoint -Authentication $ExchangeAuthenticationModel -Credential $UserCredential
Import-PSSession $Session -DisableNameChecking -AllowClobber

# For each user, get the last mailbox login time
$currentIndex = 1
$startTime = Get-Date
foreach ($user in $users) {

    # Estimate time to completion
    $estimation = " "
    $now = Get-Date
    if ($currentIndex -gt 0) {
        $elapsed = $now - $startTime # how much time has been spent
        $average = $elapsed.TotalSeconds / $currentIndex # how many seconds per site
        $totalSecondsToGo = ($users.Count - $currentIndex) * $average # seconds left
        $span = New-TimeSpan -Seconds $totalSecondsToGo # time left
        $estimatedCompletion = $now + $span # when it will be complete
        $estimation = $estimatedCompletion.ToString() # readable estimation
    }

    # Show progress and estimated completion
    Write-Progress -Id 0 -Activity "Retrieving Exchange Mailbox Stats" -Status "$currentIndex of $($Users.Count), Est Completion - $($estimation)" -PercentComplete (($currentIndex / $Users.Count) * 100)

    # Get the mailboxes last logon time
    try {
        $mailboxStats = Get-MailboxStatistics -Identity $user.mail -ErrorAction Stop
        $lastLogon = $mailboxStats.LastLogonTime
        if($lastLogon -ne $null) {
            $lastLogon = (Get-Date $lastLogon).ToShortDateString()
            $user | Add-Member -Name 'mailboxLastAccess' -Type NoteProperty -Value $lastLogon
        }
    } catch {
        $user | Add-Member -Name 'mailboxLastAccess' -Type NoteProperty -Value $null
    }
    $currentIndex++
}
Write-Progress -Id 0 -Activity " " -Status " " -Completed

# Export to CSV
$Users | export-csv C:\temp\LastLogOn_Users.csv -notypeinformation -noclobber