After setting our web server to redirect all HTTP requests to the equivalent HTTPS URL, mail to Redmine started getting bounced because rdm-mailhandler.rb was failing silently with response code 1. In our case, updating the --url option to the new HTTPS URL worked fine, but the lack of error message made debugging tedious at best. Perhaps printing the unknown response code to stderr would help solve problems like this in the future.

Tested only on 2.4.3, but the code in trunk looks identical (for checking response codes).