A fix which was made and shipped with v.2.2.M4: https://github.com/SpringSource/spring-integration/pull/493/files seems to have been subsequently dropped from the GA release. E.g.:
We have confirmed with testing 2.2.M4 vs 2.2.RC1 that when we use the former a failed transaction leaves the original mail in our POP3 mailbox, but when we use the latter it's deleted. All we change is the version of Spring Integration in our maven pom.xml to get this to happen.