On the QB master server, http_proxy, https_proxy settings were all done,
and it was well applied to QB service as well, because it was visible through master's System Attribute in Grid tab.

However, Slack plugin did not use this proxy setting, and sent a request to slack directly and failed.
Why is the server's proxy setting not used by Slack plugin?

l think server's setting should be applied and used by Slack or any other plugin that delivers something from QB to somewhere.

Please check this point out.

p.s. Slack message sending step was run on QB master as well.