Add setting to override local hostname for sending mails
Java automatically uses the local hostname for sending mails and uses it for EHLO. When running in a containerized environment, the hostname of the container might be used internally only and will not be resolvable by target mail servers. This should not cause problems with mail sending but can make tracing in case of problems harder.
A new setting should be added to override the local mail hostname. For Docker Compose, the hostname of the webserver can be reused.