We've recently had a spate of queries from authors who use Gmail who haven't received rejections that we've sent them.
I suspect that this means that Gmail is considering email from our database system (which is what we use to send the rejections) to be spam.
So if you use Gmail, and you're expecting a response from the Strange Horizons fiction department, please search your spam folder for [FICTION SUB] (without the brackets).
More generally, a lot of email gets falsely marked as spam in most email systems. So regardless of what system you use, if you're expecting a message from somebody, stop by your spam folder every now and then and do a search to see if their message got marked as junk.
Sadly, it's also true that quite a bit of email just disappears into the void these days, without a trace. So even if something doesn't show up in your spam folder, it's still possible that the message was sent but never received.
But checking the spam folder is a good place to start.