Set-SharedMbxOoo.ps1

<#
.Synopsis
   Backups up and sets the Out of Office message for all shared mailboxes
.DESCRIPTION
   Taking a path plus message as input, this cmdlet gets all current mailboxes in specific OU, loops
   through and takes a backup of (if any) Out of Office message plus parameters defined for the message.
   In the same loop, the message inputted earlier is then defined as a Enabled, External Audience Out of
   Office message.
.EXAMPLE
   Set-SharedMbxOoo -Path C:\Temp\BackupOfOOO\ -Message "Hello there!"
.NOTES
   Version:        1.0
   Author:         James Pettigrove
#>
function Set-SharedMbxOoo
{
    [CmdletBinding()]
    [Alias()]
    [OutputType([int])]
    Param
    (
        # Param1 help description
        [Parameter(Mandatory=$true,
                   ValueFromPipelineByPropertyName=$false,
                   Position=0)]
        [string]$Path = $PSCommandPath,

        # Param2 help description
        [Parameter(Mandatory=$true,
                   ValueFromPipelineByPropertyName=$true,
                   Position=1)]
        [string]$Message
    )

    Begin
    {
        #Load Exchange Powershell Module
        if (! (Get-PSSnapin Microsoft.Exchange.Management.PowerShell.E2010 -ErrorAction:SilentlyContinue) )
        {
            Add-PSSnapin Microsoft.Exchange.Management.PowerShell.E2010 -ErrorAction:Stop
        }
    }
    Process
    {
        #Get all Shared Mailboxes and loop through each mailbox
        $Mbx = Get-Mailbox -OrganizationalUnit "OU=Shared Mailboxes,OU=Users,DC=contoso,DC=co"
        ForEach ($M in $Mbx)
        {   
            Write-Host "Backing up auto-reply for shared mailbox:" $M.DisplayName
            Get-MailboxAutoReplyConfiguration -Identity $M.Alias | Export-Csv -Path "$Path$($M.Alias).csv" -NoTypeInformation

            Write-Host "Setting auto-reply for shared mailbox:" $M.DisplayName
            Set-MailboxAutoReplyConfiguration -Identity $M.Alias -ExternalAudience All -AutoReplyState Enabled -ExternalMessage $Message
        }
    }
}

Set-SharedMbxOoo