See the Bugzilla Report on this

One of the things I like about Outlook, is the fact that I can have Exchange, (or Zimbra) as my main Email account, and then add in additional Email Accounts, and create folders in my Inbox to reflect those additional accounts, then create a rule that automatically puts them in there. With Zimbra Desktop, I can have my main Inbox, but then any external Email accounts that I create, end up in an additional folder on my computer (so it's not backed up on the server). There's no way to specify to dump those accounts in my Inbox. While I could do this on the Server, the Server doesn't allow me to specify external SMTP, so I can't reply through the same email account that I received the email from.

The change I'm requesting is very simple, have a way to specify the inbox that the email goes into on the Desktop, Just like you have that option available in the Server settings, while retaining the ability to use an external SMTP.