$UserCredential = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection
Import-PSSession $Session -DisableNameChecking
Take a look if it is True: Get-TransportConfig (if there is True set to false)
Set-TransportConfig -SmtpClientAuthenticationDisabled $false
Take a look again if it works: Get-TransportConfig
Remove-PSSession $Session