Server Fault is a question and answer site for system and network administrators. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top


I have a working Postfix configuration, but no Postfix admin. Mail is being correctly delivered to all recipients. Spam filtering, etc. all work fine.

I want to configure Postfix to share a single SMTP domain and do the following: - if the receipient exists locally (e.g. if has a local maildrop), continue to deliver mail as it now does - if does not exist, forward the mail to an Exchange server

Suggestions and thoughts on the best way to make this work?

share|improve this question

I'm not a hardcore Postfix admin, but I'm thinking that the "fallback_transport" configuration parameter (see is what you're probably looking for.

You'll want something like:

fallback_transport = smtp:[x.x.x.x]

Have a look at the "local_recipient_maps" parameter, too ( You're going to want to set that to blank to stop Postfix from rejecting mail from unknown local recipients.

If you want the complementary function on the Exchange side (so that replies from Exchange to emails sent by Postfix recipients "go through") have a look at the "Forward all messages with unresolved recipients to host" setting in Exchange 2003 (see, or the "Internal Relay Domain" feature in Exchange 2007 (see

You do run the risk of creating mail loops between Exchange and Postfix for messages sent to unknown local recipients from either mail system, but the messages will time out so, unless a lot of users are generating bogus traffic, it probably won't matter too much.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.