According to http://kb.vmware.com/selfservice/mic...nalId=1004070:

The ability to send mail may be restricted so that anonymous email is not allowed or that specific user accounts are required to send mail. If this is the case a rule must be created to allow the VirtualCenter Server to send email, as there is currently no way to configure SMTP authentication with VirtualCenter.

Is there a way to configure Zimbra to allow just 1 account for vCenter to use to send email without having to authenticate?