It’s surprising to find out that there are no email notifications for release pipeline in TFS 2017. The feature is available in TFS 2018.
Instead, you have to go to azure devops market place, and find a 3rd party extension , Send Email. This extension works well, and allow you to use system and/or user variables defined in the pipeline. So you can implement a ‘Send Email’ task like this template.
Subject: RE: Deployment of release $(Release.ReleaseName) $(agent.jobstatus) in $(Release.EnvironmentName)