Powershell Move Files In A Folder

My code below is part of a larger script that loops through files in a folder and sends an email with attachment.  After each email, I would like the file to be moved to a new folder.  I am getting an error that a file is in use.  It seems to loop through all files ok, but one can never be used because it is in use.  Short code and Error below with full code.

PS C:\Users\scott> $Path = "C:\temp\"
$NewPath = "C:\temp\sent"
$files = Get-ChildItem C:\temp\*.*
ForEach ($file in $files) { 
    $attachment = $path+$file.name
  #send mail
    Move-Item $attachment $NewPath -force
}

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ERROR

Move-Item : The process cannot access the file because it is being used by another process.
At line:6 char:5
+     Move-Item $attachment $NewPath -force
+     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : WriteError: (C:\temp\test2.txt:FileInfo) [Move-Item], IOException
    + FullyQualifiedErrorId : MoveFileInfoItemIOError,Microsoft.PowerShell.Commands.MoveItemCommand

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I have also tried putting the move command after the loop.  It always leaves one file.

PS C:\Users\scott> $Path = "C:\temp\"
$NewPath = "C:\temp\sent"
$files = Get-ChildItem C:\temp\*.*
ForEach ($file in $files) { 
    $attachment = $path+$file.name
  #send mail
    
}
Move-Item $attachment $NewPath -force

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My full code is

$Path = "C:\temp\"
$NewPath = "C:\temp\sent"
$files = Get-ChildItem C:\temp\*.*
ForEach ($file in $files) { 


    $SMTPServer = "smtp.example.com"
    $SMTPPort = "587"
    $Username = "me@example.com"
    $Password = "abc134"

    $to = "you@example.com"
   
    $subject = "Email Subject"
    $body = "Insert body text here"
    $attachment = $path+$file.name

    $message = New-Object System.Net.Mail.MailMessage
    $message.subject = $subject
    $message.body = $body
    $message.to.add($to)
    #$message.cc.add($cc)
    $message.from = $username
    $message.attachments.add($attachment)

    $smtp = New-Object System.Net.Mail.SmtpClient($SMTPServer, $SMTPPort);
    $smtp.EnableSSL = $true
    $smtp.Credentials = New-Object System.Net.NetworkCredential($Username, $Password);
    $smtp.send($message)
    #write-host "Mail Sent"

  # Move-Item $attachment $NewPath -force

}

Move-Item "C:\temp\*.*" $NewPath -force

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