There are 2 config.ini files that come with release 1.4.21:
  $ find . -name config.ini
  ./share/roundup/templates/devel/config.ini
  ./share/roundup/templates/responsive/config.ini
The [nosy] section for both of these files contains the 
messages_to_author attribute. The comments above the attribute describe 
the yes, no and new settings. The comment should add a description for 
the nosy setting too. Here is what is there now:

# Send nosy messages to the author of the message.
# Allowed values: yes, no, new
# Default: no

Please consider changing the messages_to_author comment (both files) to 
this:

# Send nosy messages to the author of the message.
# Allowed values: yes, no, new, nosy
#   yes - always
#   no - never
#   new - only when the issue is created
#   nosy - only when the author is in the nosy list
# Default: no

I stumbled on the nosy option when I was comparing old code (1.4.8) for 
roundup/roundupdb.py IssueClass.nosymessage to the new code. I made a 
fix in that old version so that if one replies to a nosy email it goes 
to the sender, not the tracker. It looks like it still works that way.