pipeline-model-definition-plugin | Pipeline Model Definition Plugin provides a config-like | BPM library

When using the Jenkins pipeline where each stage runs on a different agent, it is good practice to use agent none at the beginning:

I'm trying to call a remote job from one Jenkins server to another, I have this working fine via a shell script. However, trying to translate it into a Jenkinsfile is causing me issues. The environment variable is always "null" when used inside of a stage, even thought this article says it should be globally available?

I started using Jenkins declarative pipelines. Now I want to have the same email notification behavior as defined in the Usage of the Mailer plugin:

1. Every failed build triggers a new e-mail.
2. A successful build after a failed (or unstable) build triggers a new e-mail, indicating that a crisis is over.
3. An unstable build after a successful build triggers a new e-mail, indicating that there's a regression.
4. Unless configured, every unstable build triggers a new e-mail, indicating that regression is still there.

I read about Notifications in Pipelines, but it does not notify based on above rules. Plus it does not contain part of the console output in the message body in case of a build failure.

Does anybody know how to do this in declarative pipeline?