Dear sirs!

I have zimbra 8 FOSS installation and internal domain mail.local. External mail is received with external accounts in user
preferences. Each user has another persona with external email (e.g. -
each user has two addresses, i.e user1@mail.local and
@mail.local is for confidential internal mails.
Is it way to prevent outgoing mail from addresses @mail.local to any address except @mail.local, but permit to any addresses ?

user1@mail.local -> --- may cause an error or delivery failure message -> --- everything OK
user1@mail.local -> user2@mail.local --- everything OK