When using a global environment variable, defined in "Manage Jenkins > Configure System > Environment Variables" (e.g. "ALERT_EMAIL_LIST"), in the "Email Notification > Recipients" list of a maven 2/3 project (e.g. "${ALERT_EMAIL_LIST}") the environment variable is not resolved. In the console output we see: "Sending e-mails to: ${ALERT_EMAIL_LIST}@test.com".

Note that it DOES work in an "free-style software project".

So, it does not work within <hudson.maven.reporters.MavenMailer>, but it does work within <hudson.tasks.Mailer>.