Email lists managed by the current list mailets may cause a mail loop seriously affecting all list members.
The reason is that the Return-Path header is not used according to its specification and in addition it is used inconsistently. In some cases it is used to determine the reverse path instead of using the reverse path stored along the email in the Mail class.
The correct solution would be to eliminate the dependency on the Return-Path header. E.g. it should not be used to determine the reverse path, the smtp server should not add this header, but either LocalDelivery or a specific mailstore should.)
I have a quick workaround which prevents the mail loop in the first email, the source code is in the first email of the above thread.