email redirection service with reply path through local server

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http://serverfault.com – We need to keep a copy of all email conversations on our servers for regulatory compliance. We have contractors that (understandably) want to use their own email accounts We would like to setup a system where email is forwarded to contractor inboxes, but when they reply that message needs to a) appear to come from our server and b) be saved on our server. I am sure that it would be possible to implement the following workflow, but I don't want to reinvent the wheel. Does this exist as a product/solution already? 1) Message from customer arrives in [email protected] email box 2) Service sen (HowTos)