Find everyone a mail's addressed to by loading an MSG file from MS Outlook

Get-MailRecipients

function Get-MailRecipients {
    [CmdletBinding(DefaultParameterSetName = 'ByFilename')]
    Param (
        [Parameter(Mandatory = $true, ParameterSetName = 'ByFilename', ValueFromPipeline = $true)]
        [string]$Path 
        ,
        [Parameter(Mandatory = $false, ParameterSetName = 'ByFilename')]
        [PSObject]$Outlook = $null 
        ,
        [Parameter(Mandatory = $true, ParameterSetName = 'ByMessage')]
        [PSObject]$Message 
    )
    Begin {
        $closeOutlook = $false #only close outlook if we openned it
        if (($PSCmdlet.ParameterSetName -eq 'ByFilename') -and ($Outlook -eq $null)) {
            $Outlook = (New-Object -ComObject 'Outlook.Application')
            $closeOutlook = $true
        }
    }
    Process {
        if ($PSCmdlet.ParameterSetName -eq 'ByFilename') {
            $Message = $Outlook.Session.OpenSharedItem($Path)
        }
        $Message.Recipients | ForEach-Object {
            $_.PropertyAccessor.GetProperty("http://schemas.microsoft.com/mapi/proptag/0x39FE001E") #see https://stackoverflow.com/a/45296809/361842 for mapping between properties & their schemas
        }
    }
    End {
        if ($closeOutlook) {
            $Outlook.Quit()
        }
    }
}