Thunderbird has a bug whereby it doesn't set the INTERNALDATE when using APPEND to copy messages. You can work around this by putting "
/tb" at the end of your login username (e.g.
user@domain.com/tb or
user/tb), which throws the Zimbra IMAP server into "
Thunderbird hack mode" for your session and works around this issue by setting INTERNALDATE to the
Date header of APPENDed messages.
Nothing can be done about the messages you've already copied over, however.