Is there any way to return email as though it was undeliverable or the mailbox is full?
I can see how this would be helpful. Got invited to a baby shower you don’t want to go to? Somebody asked you to help them move? It’d be nice to deliver a fake response like “Email returned to sender because mailbox was full.”
If the sender is dumb enough, you might be able to fake it by forwarding it back to them, but I doubt it. Most email servers have their own automated message for undeliverable mail. And if the sender is smart enough and suspects you were just backing out of something, he could check the headers and see that you really just forwarded the message (I think… not really sure how headers work, but I know they’re often used to verify emails.)
Your best bet is probably to just pretend you never saw it because you didn’t check your email box. Or you can say that your email server accidentally, automatically picked it up as spam and placed it in the spam folder. Or you can just lie to get out of it can’t you?
OR, I assumed the wrong motive… so why do you want to do this?
Art
I always help people move when they ask, it goes so much more quickly with even one extra pair of hands. Baby showers are easy to turn down, as long I send a nice gift.
I need to find a way to bounce emails that my ex-boyfriend keeps sending me. Answering them or ignoring them doesn’t seem to get him to stop sending them, and he keeps asking mutual friends if I’ve read them, which is unpleasant for them. The problem is that he has a B.S. in comp. sci. 
In order to do what you want you need access to the mail server, which you don’t have.
Set up an auto-reply so that any incoming mail from his account will be answered with a form letter saying the messages will not be read and will automatically be deleted (you’ll probably want to set it up so they automatically will be deleted too). After the first hundred or so copies he gets of this form letter he should get the hint.
Sounds like you have more of a stalker problem than a technology problem though, and I can only help with the techie part.
Hmmm… well you got me there. If you block an address, do they get their mail returned to them? I’m not sure since I’ve never found a need to do this, and since I’ve started using gmail I don’t even open outlook express anymore; so I don’t even have the option of blocking email. If you have outlook, you could always email yourself from a different addy, block yourself, and see if you get a returned message to find out.
OR, you can just confront this guy. He obviously checks his email, you should just send him an email telling him that you will no longer be checking his messages so he would only be wasting his time to send them. That way you don’t even have to confront him face to face. Avoiding confrontation is a way of life.
OR, somebody more clever will come along to answer your question better than me. 
Art
FWIW (probably not much) Mac OS X Mail has a “bounce selected messages to sender” option that makes the e-mail look like it was undeliverable. I’d think that if Mac can do it, there’s got to be at least one Windows mail program that can do the same.
I believe that program will do it. It’s free.