that's right -- it won't inform the sender. Please realize that this level of error handling would quickly overwhelm our servers and that holding off on this soft of thing is a big part of why we're able to stay free (and even so, I'd be interested to learn of a commercial service that manages to do this). Look at the stats -- >90% of email reaching sg is spam. Most of those messages are sent by forging bots -- if they're warned, it'll generate more bounces. We may be able to build in logging in the future, but that will be challenging as well -- sg partially "remails" the message to the forwarding address, and so a bounce comes back to a single system address with no easy way to tell which account it came from -- this address also receives a *lot* of other messages, almost all of which are meaninglessly generated by the various unsavory automated processes at work in the digital underworld. It will require quite a bit of CPU to separate the tiny signal from the enormous noise.
Have you tried temporarily switching the forwarding address for your account? That can isolate any problems with the current one. Get a gmail address (I've got 6 invitations to give out, if you need one
) or a yahoo account or something for this purpose.