When user leaves orginization administrators make sure that account is disabled and its marked for deletion. Delection can happen once in 15 days or 1 month.  We may need to perform series of steps for disabling the account

eg. Disable Account, Move Object to Disabled Account OU, Hiding from GAL, removing Group members, 0 ing send and receive limits.

Below powershell script helps to perform the same.  It uses both Exchange commands and Quest Active roles command lets. We need to add the snapin to execute the code.

Add-PSSnapin Microsoft.Exchange.Management.PowerShell.Admin
Add-PSSnapin Quest.ActiveRoles.ADManagement
$AName = Read-Host “Enter User Alias name for Disable”
$AName | out-file -filePath E:\users.txt
foreach ($user in (get-content E:\users.txt)){(get-qaduser $user).memberof | Get-QADGroup | where {$_.name -ne “domain users”} | Remove-QADGroupMember -member $user}
Move-QADObject $user -NewParentContainer “domain.com/Disabled Accounts”
Disable-QADUser $user
Set-Mailbox $user  -HiddenFromAddressListsEnabled $true -UseDatabaseQuotaDefaults:$False -issuewarningQuota 0MB -ProhibitSendQuota 0MB -ProhibitSendReceive 0MB

Below location has copy of the code

http://powershell.com/cs/members/smtpport25.wordpress/files/DisableUserAccounts.ps1.aspx