try following as zimbra user:
Code:
/opt/zimbra/bin/zmmailbox -z -m mailbox1@domain.com getRestURL "//?fmt=tgz" > /tmp/mailbox1.tgz
/opt/zimbra/bin/zmmailbox -z -m mailbox2@domain.com postRestURL "//?fmt=tgz&resolve=modify" /tmp/mailbox1.tgz
this would merge mailbox1's mails into mailbox2
Dipesh