Does zimbra send automatic reminder emails?

I am in the process of testing zimbra server and zimbra desktop internally with a few users. I have also configured a couple of the zimbra desktops with our external accounts. Every so often it seems to send a reminder from the default account to the default account, which on all the zimbra desktops is the external account. It gives a send error that shows up in the external account. In both the internal and external account there is no trace of the email being sent. I need to turn this

From: assistenza@external server
To: assistenza@external server
Message-ID: <21372056.7.1301385237224.JavaMail.l.mussorofo@ASS ISTENZA1>
Subject: Delivery failed: Invalid Addresses
MIME-Version: 1.0
Content-Type: multipart/mixed;

The server is zimbra 7 opensource on ubuntu 10.04 and the Zimbra desktops are both 2 and 7 beta 1.