I went through the same experience a couple of months ago while trying to send messages to Bob's "engineering" email address. Each one came back with an error message. It turns out that the messages were actually going through despite the warning.
At the time, at least, Bob was absolutely swamped, to the point that he was working for 40 hours at a time without ever even getting to his email. I'm sure he'll respond when he can, but hopefully not at the expense of 40 hour "days".