Issue Summary

Sending a POST request to the /rest/api/3/issue/{issueIdOrKey}/notify REST API endpoint returns status code 500 (Internal Server Error) instead of 400 (Bad Request) in case of errors in the request body.

Steps to Reproduce

1. Send a POST REST API request to the with below body to the issue notify endpoint *https://SITENAME.atlassian.net/rest/api/3/issue/ISSUE-KEY/notify*

   { 
   "subject": "Test Email", 
   "textBody": "Body", 
   "htmlBody": "<h1>Body</h1>", 
   "to": 
     { "voters": false, 
       "assignee": true, 
       "watchers": false, 
       "reporter": true
     }
   }
   

2. See that the request works fine (returning status code 204) and that assignee and reporter of the issue having key  ISSUE-KEY receive a notification as expected.
3. Now send below (wrong) body to the same endpoint:

   { 
   "subject": "Test Email", 
   "textBody": "Body", 
   "htmlBody": "<h1>Body</h1>", 
   "to": 
     { "voters": false, 
       "assignee": false, 
       "watchers": false, 
       "reporter": false, 
       "groups": null 
     }
   }
   

Expected Results

Status code 400 (Bad request) is returned

Actual Results

Status code 500 (Internal Server Error) is returned instead

Workaround

No workaround available at the moment.