Powershell script to apply retention policy by group - Exchange 2010

Question

I am working on a script to apply a specific retention policy to individuals who are members of a particular group, another for rooms, and yet another to everyone else.  Below is what I'm working with:
I run into an error that states "Pipeline not executed because a pipeline is already executing..." and the retention policy doesn't get set.  can someone tell me where I've gone wrong here?  Or perhaps an easier solution?
#Check entire domain
Set-ADServerSettings -ViewEntireForest $true
#Active Directory Group
$Group = [ADSI]("LDAP://CN=EXCHANGE_DISABLE_PURGE,OU=EXCHANGE ACCOUNTS,DC=domain,dc=com")
#Enum Group Members and set equal to array
$Grouplist = $Group.member
#do loop varaible setup
$GroupLength=$Grouplist.count
$GroupArrayLength=$GroupLength - 1
$GroupCount = 0
$Userlist= @(0..$GroupArrayLength)
$UsersDN=@(0..$GroupArrayLength)
#do loop to disable purge policy for all array members
do {
$Userlist[$GroupCount] = [ADSI]("LDAP://" + $Grouplist[$GroupCount])
$UsersDN[$GroupCount] = $userlist[$GroupCount].distinguishedname
$GroupLength=$GroupLength - 1
$GroupCount++
until ($GroupLength -eq 0)
Get-mailbox -database db01 -resultsize Unlimited | foreach-object{
if ([string[]]$UsersDN -contains $_.DistinguishedName){
  if ($_.RetentionPolicy -ne "NoPurge"){
   set-mailbox $_ -RetentionPolicy NoPurge
   write-host("Applied - No Purge Policy to "+$_.DisplayName) -ForeGroundColor Green
elseif (($_.RecipientTypeDetails -eq "RoomMailBox") -or ($_.RecipientTypeDetails -eq "EquipmentMailbox")){
  if ($_.RetentionPolicy -ne "Default Resource/Equipment Policy"){
   set-mailbox -identity $_ -RetentionPolicy "Default Resource/Equipment Policy"
   write-host("Applied - Default Resource/Equipment Policy to "+$_.DisplayName) -ForeGroundColor Green
else{
  if ($_.RetentionPolicy -ne "180DayDefault"){
   set-mailbox -identity $_ -RetentionPolicy "180DayDefault"
   write-host("Applied - Default 180 Day Purge Policy to "+$_.DisplayName) -ForeGroundColor Green

Answer 1

Hi,
I generally get all mailboxes first and then use a foreach loop to process each one. This usually seems to fix the error about concurrent pipelines.
$mbxs = Get-Mailbox
foreach ($mbx in $mbxs) {
# Do stuff here on each mailbox
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