I wonder if any thought has been given to providing users with the ability to allow external users not only to view the contents of a briefcase, but also to write to it. Perhaps on a permission-per-transaction basis?
It seems to me that with this capability, Zimbra could be a good alternative to the type of service provided by Accellion.
While email can be made secure by doing SMTP over TLS, for example, it's not currently practical to force this for all connections. However if I give my users the ability to create an externally-writeable briefcase on the fly, assign rights to specific outsiders, and if I require https for this access to zimbra--I think that would pretty much do the trick.
I'm less concerned about the issue of large file transfer than about security, but, subject to the load a briefcase upload puts on the system, this same approach would let me just add more storage to my zimbra system as needed.
Right now the main alternative of which I'm aware would be an sftp server, which involves a lot of overhead as users request changes to client access.