For a long time, FastMail has had a "bounce" action in the web interface. When applied to email(s), it would generate a standard looking "bounce" email back to the sender, making it look like the original email delivery had failed.
This feature has now been removed from the web interface because it now causes many more problems than it solves.
The problem is that when the feature was first created, most emails had valid "From" addresses. These days the "From" address on most unwanted emails are forged and random, so bouncing an email really just generates backscatter emails, which themselves are considered spam. So rather than stopping spam, the bounce feature actually just generates more spam to other people!
On top of that, the bounces can cause our servers to be listed on IP blacklists. For instance, a user bouncing a large number of messages over a couple of days several months back caused most of our outgoing server IPs to be listed at http://www.backscatterer.org/. Having any of our outgoing server IPs listed on any RBL blacklist is a bad thing, because there are always systems out there using obscure RBLs like this (and worse, sometimes incorrectly configuring them to reject all email rather than just postmaster \<> empty address bounce emails), which then means other users get emails they're trying to send bounced, which is always bad.
Although there are one or two cases where bouncing might be useful, they're rare compared to what people are actually doing, and these days, it's much better to do the following actions: