I wrote about Robin Fundinger's question,
| The current behavior are correct.
Oops. It are, but the subject-verb agreement were not. Sorry.
In any case, there is something I left out: if Monica wants to save Ken's
address in another header so that Bill can find it out if he *does* receive
his email, that's all right. But bouncing mail back to Ken if Bill cannot
get the delivery is not the right thing to do.
To summarize, if Monica is using an invalid or obsolete or temporarily
unavailable address for Bill, you don't tell Ken. You tell Monica. The
current behavior is correct.