well maybe not so much hide it as have it be something else...we are using another mail server as our main mail server, we forward their alias "user@theirdomain.com" to an email account they have setup with us "user@ourdomain.com"
the ideas was to have ourdomain.com forward to
user@zimbra.theirdomain.com so that they can utilize the collaboration functions of the calendar, etc...but if the user wants to send directly from zimbra web access or desktop, it would be preferable for the users address to not show as @zimbra.domain.com, I would rather it simply show as if it is coming form the users original alias.. sorry I know it is a bit convoluted..