535 5.7.3 Authentication unsuccessful
You need to set SmtpClientAuthenticationDisabled to false.
get-CASMailbox -Identity user@domain.com | fl SmtpClientAuthenticationDisabled Set-CASMailbox -Identity user@domain.com -SmtpClientAuthenticationDisabled $false
Server: smtp.office365.com Poort: 587 Versleuteling: STARTTLS