This is interesting. I've just attempted to send myself an email (G Suite -> Xtra Mail) and... wouldn't you believe it? Blocked as spam. *sigh* Either too little or too much filtering. What's worse is that the message contains only "Testing 123. If you can see this, Spark isn't blocking your own mail", so there's exactly 0 chance it should be detected as spam. Oh well. Time to get the parents (and grandparents) to stop using Spark's own email.