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One thing you can do if you have several domains, and the same users in those domains, is to create a domain alias. For example, at my company here we have two domains, domain1.com and domain2.com - all users have the same user@ address in both domains. domain1.com is our primary domain, but anyone who sends an email to
user@domain2.com will have their email sent to
user@domain1.com's email inbox. The recipient's address is
not rewritten using this method, but will be delivered to the proper mailbox.
ManagingDomains - Zimbra :: Wiki