Office 365: Change default e-mail retention to 1 year

<#
.SYNOPSIS
This script creates a new retention tag and adds it to the Default MRM Policy in Exchange Online.
It correctly removes any existing 'Default' tag from the policy.

.DESCRIPTION
1. Creates a "Default" retention tag that moves items older than 1 year (365 days) to the archive.
2. Finds the "Default MRM Policy".
3. Finds and removes the *existing* 'Default' tag from that policy.
4. Adds the new 'Default' tag to the policy.

.NOTES
You must be connected to Exchange Online PowerShell before running this script.
(Use Connect-ExchangeOnline)
#>

# --- Connect to Office 365 ---
Connect-ExchangeOnline

# --- Step 1: Define and create the new tag properties ---
$TagName = "Default 1 year move to archive"
$AgeLimit = 365 # 365 days = 1 year
$TagAction = "MoveToArchive"
$TagType = "All" # This applies to all untagged items in the mailbox

Write-Host "Checking for existing retention policy tag: $TagName..."
$ExistingTag = Get-RetentionPolicyTag -Identity $TagName -ErrorAction SilentlyContinue

if ($null -eq $ExistingTag) {
    Write-Host "Tag not found. Creating retention policy tag: $TagName..."
    New-RetentionPolicyTag -Name $TagName -Type $TagType -AgeLimitForRetention $AgeLimit -RetentionAction $TagAction
    Write-Host "Successfully created tag."
} else {
    Write-Host "Tag '$TagName' already exists."
}
Write-Host "---"

# --- Step 2: Add new tag and remove old default tag from the Default MRM Policy ---
$PolicyName = "Default MRM Policy"

Write-Host "Updating policy '$PolicyName'..."

# Get the policy
$Policy = Get-RetentionPolicy -Identity $PolicyName
if ($null -eq $Policy) {
    Write-Host "ERROR: Policy '$PolicyName' not found."
    # Use 'break' or 'return' if this is part of a larger function
    return
}

# Get all retention tags in the organization
$AllTags = Get-RetentionPolicyTag

# Get the names of the tags currently on the policy
$PolicyTagNames = $Policy.RetentionPolicyTagLinks

# Get the full tag objects for the tags on the policy
$PolicyTags = $AllTags | Where-Object { $PolicyTagNames -contains $_.Name }

# Find the *old* default tag (one that is 'All' type but not our new tag)
$OldDefaultTag = $PolicyTags | Where-Object { $_.Type -eq 'All' -and $_.Name -ne $TagName }

# Create the new list of tags for the policy, starting from the current list
$NewTagList = $Policy.RetentionPolicyTagLinks

if ($null -ne $OldDefaultTag) {
    Write-Host "Found existing default tag: $($OldDefaultTag.Name). It will be removed."
    # Remove the old default tag from the list
    $NewTagList = $NewTagList | Where-Object { $_ -ne $OldDefaultTag.Name }
} else {
    Write-Host "No other 'Default' tag found to remove."
}

# Add the new tag to the list (if it's not already there)
if ($NewTagList -notcontains $TagName) {
    Write-Host "Adding new default tag: $TagName"
    $NewTagList = $NewTagList + $TagName
} else {
    Write-Host "New tag '$TagName' is already on the policy."
}

# Set the policy with the updated list of tags
Write-Host "Applying new tag list to policy..."
Set-RetentionPolicy -Identity $PolicyName -RetentionPolicyTagLinks $NewTagList

Write-Host "Successfully updated policy '$PolicyName'."
Write-Host "---"
Write-Host "Script completed."