$credentials = Get-Credential -Credential pnovotny@lotraco.cz
Write-Output "Getting the Exchange Online cmdlets"

$Session = New-PSSession -ConnectionUri https://outlook.office365.com/powershell-liveid/ `
    -ConfigurationName Microsoft.Exchange -Credential $credentials `
    -Authentication Basic -AllowRedirection

Import-PSSession $Session

Get-Mailbox -RecipientTypeDetails UserMailbox -ResultSize Unlimited | Foreach-Object{

    $mbx = $_ | Select ExchangeGuid, UserPrincipalName, DataBase

    $userFolder = $_.alias +":\Kalendář"
    Set-MailboxFolderPermission -Identity $userFolder -User Default -AccessRights Reviewer
}