A client has an Exchange 2010 infrastructure in place, with a primary email address of company.net. They also have a separate department that has a Linux based mail server in it, with an address space of eng.company.net. They need to send email through the Exchange server to internal and external recipients and they also need to accept email for this Linux server. The server name is elrond.eng.company.net and they want to accept mail for mailman.eng.company.net. I have the Receive Connector working, so the mail can be sent out. However, the messages sent out are rejected because there are no external DNS entries for this Linux server. What external entries do I need to create to make this work? I assume that I will need to create a new external DNS zone of eng.company.net so that I can create the MX record for it.

If you are receiving email from the Internet for that sub domain then you already have a zone. As far as the Internet is concerned, example.com and sub.example.com are two different domains and require two different zones. The host name for the MX record can be the same. Therefore if you are routing email for those two domains in via Exchange then you can use the host name of the Exchange server (ie the MX record host name for the main domain) as the MX record for the sub domain. For sending email out, what you are probably missing is a PTR, aka reverse DNS. That needs to be set by your ISP and would require the Linux system to have its own external IP address. If you are going to give it a unique external IP address, then you can probably skip having external email delivered through Exchange and have it delivered directly. If you don't have a spare IP address, then the Linux server will need to be configured to send email via the Exchange server, and you will need to configure a connector to allow that server to relay. Technet has an article on application server relaying which is effectively what you are doing. Simon.Simon Butler, Exchange MVP Blog | Exchange Resources | In the UK? Hire Me.

I was able to get this configured. Created the appropriate MX records and had the ISP do the reverse record. Seems to work just fine.