It changes the logins for all the services that Zarafa provides itself: WebApp, IMAP, CalDAV, CardDAV, Outlook. It doesn't change other services, though, and SMTP is one of those other services as it is provided by postfix and not by Zarafa.
If you want to adjust Postfix to use the email address as the login name, too, for authenticating users and allowing them to forward emails you'll have to configure the SASL authentication daemon (saslauthd). Postfix itself only passes the username & password provided by the client to the saslauthd, and saslauthd in turn is configured to use the system accounts by default. The system accounts use the usernames you do not want to use instead of primary email address.
However, you can configure saslauthd to authenticate against an LDAP server instead, and that should allow you to use the email address as the attribute used in user lookups. See "man saslauthd".
Note that the postfix issue is not limited to Zarafa. It also applies to other groupware solutions.