Param( #Defines Parameters that will be used within this script.
    [Parameter(Mandatory=$true)] #This is an attribute applied to the parameter. It specifies that a parameter is required. 
    [string]$userToAddAccess #[String] is the type of parameter. $userToAddAccess is the name of the parameter.
)

Connect-ExchangeOnline

#This grabs all USER mailboxes and not SHARED
$mailboxes = Get-Mailbox -ResultSize Unlimited -RecipientTypeDetails UserMailbox | Where-Object { $_.RecipientTypeDetails -ne "SharedMailbox" }

#This iterates through each mailbox using a FOR Loop and grants '$userToAddAccess' full access to the user mailbox ($mailboxEmail)
foreach ($mailbox in $mailboxes) {
    $mailboxEmail = $mailbox.UserPrincipalName
    Add-MailboxPermission -Identity $mailboxEmail -User $userToAddAccess -AccessRights FullAccess -AutoMapping $false
    #Prints a list of the current delegation of user's mailbox. This is so you can check delegation as the script works through the list of user mailboxes
    Get-MailboxPermission -Identity $mailboxEmail | Where-Object { $_.AccessRights -like "FullAccess" }
}

# Disconnect from Exchange Online
Disconnect-ExchangeOnline