1) A quick solution, create /opt2/mail, then move all files under /mail/ to /opt2/mail, remove empty directory /mail, create symbol link /mail which linked to /opt2/mail.
2) A hard solution: create /opt2/mail, move all files under /mail/ to /opt2/mail, update LDAP attribute 'homeDirectory' and 'storageBaseDirectory' to replace /mail/ by /opt2/mail/.
IMPORTANT NOTE: Directory used to store mailboxes must be owned by vmail/vmail with file permission 0700.