[Exim] Help: How do I identify virtual domains and users

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I have several domains that I host, ie (domiain1.com, domain2.com, domain3.net, etc) and I have users associated with each one of this domains i.e (bob@???, john@???, mary@???, etc). I am runing a exim 4.30-6 debian distribution woody and I can successfully send mail to all of my virtual users and dominains, however when the users reply the reply address is of the form of user@??? and I want it to be user@???/net . Any help will be appreciated in advance. Thanks
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