Generally greylisting is supposed to let future emails through once it establishes that a server will re-send. It should be possible to detect greylisting and w

Generally greylisting is supposed to let future emails through once it establishes that a server will re-send. It should be possible to detect greylisting and warn the user. "Your email provider temporarily rejected our email to you. We will retry in a few minutes. If this happens persistently, please (check our guide for your provider, $MAJOR_PROVIDER|contact your provider) to figure out how to whitelist our login emails"