On the forum I saw a bunch of posts that explain how to setup an event listener and add this to the transition of the project's workflow. My goal is to do almost the same. I would like to sent an email to a custom email address when the assignee field is changed from unassigned to a person and also from a person to unassigned. The problems I faced when using solutions from other forum topics:
1) When using issue.assignee != originalIssue.assignee the event only seems to trigger from unassigned to assigned, but not on assigned to unassigned. Could be a Script Runner issue or improperly configured by me.
2) The Send a custom email was configured in the addon section, but when later trying to add it to a transition, I am unable to select the pre-configured event. The fields are all blank and it looks like I would need to configure this addon each transition post event. I have about 30 transitions 
3) Is there no way to specify the project and sent an email based upon the assignee field changing generally? Because there will be cases where there is no transition upon changing the assignee. I have a project based on this guide: https://www.atlassian.com/blog/jira-service-desk/free-ebook-jira-service-desks-guide-asset-management
Any suggestions are very welcome!
Jira version: v7.1.7#71011