There should be a note in the chapter "Distribution of an installation on several mail servers", that by changing a users mail home server its mails are not moved to the new server automatically.
Added in r60183.
OK: fixed wording. OK: Added translation. r60636 | Bug #38356: Changing of the mail home server of a user does not move its mails r60635 | Bug #38356: Changing of the mail home server of a user does not move its mails
Published r60655: UCS-4.0-2 Manual