Just an FYI...
We are running ZCS 5.0.10 and recently started getting email bouncing back from certain domains saying the email was corrupted. This only occurred for emails with attachments, plain text emails were fine.
Basically, email *from* zimbra 5.0.10 with attachments going to a domain that is fronted by a barracuda spam firewall with firmware version 3.5.11.020 (released in April 2008) was somehow corrupting the emails.
Sending the same email (w/attachments) from a gmail account to the same domain was going through fine, so there was something specific about the way zimbra constructed the headers or mime stuff that choked barracuda spam firewall.
After the admin at the remote domain upgraded to the latest firmware version for barracuda spam firewall (3.5.12.007 released in November), emails from our zimbra server are now being delivered properly. There was another version from May that is probably also ok (3.5.11.025)
The admin said that in the barracuda firmware release notes, there are no specific notes about issues with attachments but there were a few changes to the way attachments are handled. The admin only found anecdotal evidence about corruption problems with 3.5.11.020 in the Barracuda Support forum.