[Mailman-Developers] Facing error while building mailman

[Stephen J. Turnbull]

This -- "Python Version: 2.7.9" is also a problem for recent Mailman 3
(since 3.1 IIRC) -- Mailman 3 is only supported on Python 3.  I guess
this is Postorius and it's still OK for 2.7 (upgrading to the most
recent Python 2 version, which I believe is 2.7.11, is probably a good
idea, as there were security issues in 2.7.9 and 2.7.10).

I would guess that your proxy simply isn't permitted to access the
REST port by the server.  Check firewall settings and try a simple
client such as wget or curl from the Mailman host.  If you get the
same result (ie, 403 Forbidden) with those clients, then you're going
to need to talk to your local administrators about how to configure
the firewall to permit Postorius to talk to Mailman.

Steve