The problem is at WAR deploy, something is connecting to the email server, encounters a problem, and then causes a shutdown. The error says "Port number N … is in use", but nothing is running on that port.
The error occurs no matter what the port number is.
However, when removing the port email property used with Spring Boot, the problem goes away: spring.mail.port=15025 (of course, that makes the app use the production-configured values which doesn’t work for all situations)
To reproduce, run "mvn install" in empty-war dir and then in acceptance-test dir.