Private/GetManagerEmailAddress.ps1

function GetManagerEmailAddress {
    [OutputType('string')]
    [CmdletBinding()]
    param
    (
        [Parameter(Mandatory)]
        [ValidateNotNullOrEmpty()]
        [object]$AdUser
    )

    $ErrorActionPreference = 'Stop'

    if ($AdUser.Manager -and ($managerAdAccount = Get-ADUser -Filter "DistinguishedName -eq '$($AdUser.Manager)'" -Properties EmailAddress)) {
        $managerAdAccount.EmailAddress
    }    

}