I've never had this happen before.
Using the webmail interface, I just quick-reported a held-folder full of spam (145 messages) and I got about 30 bounces in my inbox that look like this:
> Hi. This is the qmail-send program at mx53.cesmail.net.
> I'm afraid I wasn't able to deliver your message to the following addresses.
> This is a permanent error; I've given up. Sorry it didn't work out.
> <ver.[my+spamcop.net.address]-[long-alphanumeric-string]@spam.spamcop.net>:
> 204.15.82.29 failed after I sent the message.
> Remote host said: 554 #5.3.0 Server Error
>
> --- Below this line is a copy of the message.
EDIT -- I just noticed I also have about the same number of bounces that got caught in my held mail folder.... DRAT...!
EDIT2 -- Just to be clear, the bounced messages are the spam from my held folder that's being bounced back to me. I just released all that had been caught so I don't accidentally report the bounces.
P.S.: If anyone has further questions for me, I'm traveling and may not see them for a day or two.
