Zimbra 6.0.7 uses a wrong "MAIL FROM" command, sending emails with external accounts

[dennydgl1]

Hi all, I post this thread because I'm facing a problem sending emails from external accounts.
I have one zimbra account, myaccount@myfirstdomain.com which has an external account configured myexternalaccount@myexternaldomain.com.
I need zimbra to relay mails sent by myaccount@myfirstdomain.com to the ISP's MTA (configured as postfix's relayhost) and the one sent by myexternalaccount@myexternaldomain.com to be relayed through another MTA (configured as sender_dependent_relayhost_maps in postfix's main.cf).
Unfortunately the mechanism does not work because if a login as myaccount@myfirstdomain.com and I send an email as myexternalaccount@myexternaldomain.com, the message has still the from field containing myaccount@myfirstdomain.com so it will be sent through the default relay host.
Could someone explain me why I get this strange behaviour?
Could it be a known issue of my Zimbra release? I use zcs-6.0.7_GA_2473.UBUNTU8_64.20100616212156.

Regards,
Denis Maggiorotto