Perhaps you are using a USR sportster or other faulty modem.
Try adding S12=0 and S2=128 to your modem initialization string (ATS12=0S2=128). This disables the escape sequence (normally - +++). I heard that this is a common problem with USR 56K's - they erroneously detect the escape sequence and go into a wrong mode.
The reason you see it with SpamCop is that the web forms get "URLencoded" which means that spaces are converted to pluses and if the spam that you tried to paste had three consecutive spaces in it, Netscape sends +++ and this kills the connection...
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