You should check the values for DEFAULT_EMAIL_HOST and DEFAULT_URL_HOST in Defaults.py. Make any necessary changes in the mm_cfg.py file, not in the Defaults.py file. If you change either of these two values, you'll want to add the following afterwards in the mm_cfg.py file:
add_virtualhost(DEFAULT_URL_HOST, DEFAULT_EMAIL_HOST)
You will want to run the bin/fix_url.py to change the domain of any existing lists.