'PowerShell Email Multiple Recipients only sending to the last

I am writing a script to send emails to a manager and employee. I am creating a string of recipients, but when the send-mailmessaage runs it only sends to the last email on the string. Thus in the example it only sends to [email protected], and not [email protected]. I need it to send to both

$recipients = ''
    if($usermanager.mail -ne $null){
    $recipients = '' + $usermanager.mail + ''
    }
    if($userengineer.mail -ne $null){
    $recipients += ', '+ $userengineer.mail + ''
    }
    

   #$Recipients now equals [email protected], [email protected] 
    
    Send-MailMessage -SMTPServer 'OUTLOOK-test.COM' -From 'test1, test1<[email protected]>' -To $recipients -Subject $subject -Body $emailBody -BodyAsHtml


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