Just looked around a bit to send an email to a list (an
external address) when a bug is set to a specific
status (like "done-cbb" or "resolved").
From what I've understood so far I can't directly use
roundupdb.send_message() as a msg is needed.

So I want to create my own message for this issue.
It would be nice to have a more general method which is
called
by send_message() to create everything that is needed
to send an email related to one issue, but does not
need a msg.

Alternatively I would appreciate an example which
solves my task
if it is possible using a temporary msg object.

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send_message is still a bit specific to nosy messages, and I doubt that  
will change any time soon (making it too generic may make it too  
complex). Have a look at the roundup-reminder script - perhaps it  
could be used for additional clues?  
  
If you do come up with something, please consider contributing a   
section for the customisation doc "examples".  If you need further 
help, please discuss on the -users mailing list.