详见https://techcommunity.microsoft.com/t5/microsoft-365/can-t-send-smtp-mail-after-setting-default-security-in-office/m-p/1181319
Connect-MsolService
Import-Module ExchangeOnlineManagement
Connect-ExchangeOnline
Get-TransportConfig | Format-List SmtpClientAuthenticationDisabled
Set-TransportConfig -SmtpClientAuthenticationDisabled $false