I've had a breakthrough of sorts! I've been using eNewsletter for 3 months now, and even though the bounce account has been receiving bounce messages, I've not once seen a bounce registered in eNewsletter - until today! And I'm so excited - heehee (the simplest things you know!).
Anyway, I don't know why it's suddenly working, but it really is. I sent out an email to about 4800 subscribers, 8 messages have ended up in the bounce account thus far, 4 of which were actually "out of office" messages, and 4 true bounces. The 4 out of office messages did not trigger a bounce in eNewsletter, but the true bounces did!
Since I'm always asking questions, I thought I'd post and let you something is working now that I had assumed would just not be that useful to me!