Issue Summary

This is reproducible on Data Center: yes

Steps to Reproduce

Setup a Jira 9.1 environment and navigate to the Jira Incoming Mail Setup screen as below

1. Goto Jira Administration>System>Incoming Mail
2. Under the "Advanced Configuration" enter any value for "Message Threshold" and save

Once, we click on save, the page returns "HTTP Status 405 – Method Not Allowed" as below ** 

Expected Results

"Message Threshold" should be saved without any issues.

Actual Results

"Message Threshold" save returns "HTTP Status 405 – Method Not Allowed"

We can see below WARN in the atlassian-jira.log file:

2022-08-01 13:49:08,901+0000 http-nio-8080-exec-26 WARN admin 829x1219x1 ttdnff 172.29.178.92,10.9.32.234 /secure/admin/IncomingMailServers!saveConfig.jspa [c.a.j.web.dispatcher.JiraWebworkActionDispatcher] Action 'com.atlassian.jira.plugins.mail.webwo
rk.ViewMailServers!saveConfig' does not support 'POST' request method 

Workaround

Currently there is no known workaround for this behaviour. A workaround will be added here when available