steve288
In case you get uppity, this is answering your question, it's just not the answer you are expecting or wanting.
Why are you worried about a few bounce messages when you should be worried about connecting an insecure sme v6.x server to the Internet ?
Do us all a favour and remove your insecure server from the Net. Please upgrade it to sme 7.3, as that version has much improved email handling & so much more etc etc etc.
You may be surprised to see that the default sme7.3 resolves many of your bounce issues.
It's a free download and will still run on all but the oldest equipment you may be using for sme6.x.
I have sme7.3 running on a P333 box with 256Mb RAM. Server manager functions are slow, but web access is acceptable. I'm not promoting that as recommended practise, just saying that sme7.x will run on older hardware, especially where only light duty is being done.