Return Path wrong if using "Persona"

[dgentile]

I run Zimbra on a Dedicated Server along with some other stuff..
Zimbra is the latest release (6.0.10...)

everything seems to work just fine - but for one rather serious problem:

As I (and some other users on my server) have multiple e-mail adresses we use one Zimbra account (for example: mail@mydomain.com) but have set different persona in each account (for example mail@mydomain.com has a persona john@gmail.com)...

But whenever I send an E-mail from a Persona (john@gmail.com) the recipient get the following e-mail with a "wrong" header:

From Daniel Gentile Wed Feb  9 19:33:44 2011
X-Apparently-To: test@ymail.com via 98.138.88.159; Wed, 09 Feb 2011 11:33:45 -0800
Return-Path: <mail@mydomain.com>
Received-SPF: none (mta1086.mail.sk1.yahoo.com: domain of mail@mydomain.com does not designate permitted sender hosts)
X-YMailISG: zFx2g...kOjOO5A--
X-Originating-IP: [xxx.xxx.xxx.xx]
Authentication-Results: mta1086.mail.sk1.yahoo.com  from=mydomain.com; domainkeys=neutral (no sig);  from=ferrumd.net; dkim=neutral (no sig)
Received: from 127.0.0.1  (HELO p3plxxx.net) (xxx.xxx.xxx.52)
  by mta1086.mail.sk1.yahoo.com with SMTP; Wed, 09 Feb 2011 11:33:45 -0800
Received: (qmail 673 invoked from network); 9 Feb 2011 19:33:44 -0000
Received: from unknown (HELO mydomain.net) (xxx.xxx.xxx.44)
  by p3pls....net (xxx.xxx.xxx.52) with ESMTP; 09 Feb 2011 19:33:44 -0000
Date: Wed, 9 Feb 2011 12:33:44 -0700 (MST)
From: John Doe <mail@mydomain.com>
To: test@ymail.com
Message-ID: <19392196.33.1297280024709.JavaMail.root@mydomain.com>
Subject: test 800
MIME-Version: 1.0
Content-Type: multipart/alternative; 
	boundary="----=_Part_32_27198870.1297280024707"
X-Originating-IP: [xxx.xxx.xxx.17]
X-Mailer: Zimbra 6.0.10_GA_2692 (ZimbraWebClient - SAF3 (Mac)/6.0.10_GA_2692)
Content-Length: 897

That actually is worse than gmails "sent-on-behalf-of" header....

My Previous zimbra hoster (I had the accounts hosted before) had Zimbra 5.xx and there was never a problem with using persona-settings...

Is there any way to get this working properly?