I have this application/website that needs to send emails, I have it configured to use our exchange server as SMTP. It can send emails to internal users however external ones are failing with a "550 5.7.1 Unable to relay" error
I have been answering this question, under different forms, so often on Experts Exchange lately that I have it copied on a notepad and end up by pasting it again every now and then.
First a mail server should be configure to unconditionally (not talking about spam/viruses here) accept emails sent to users in the smtp domains it serves. However it should be careful about who gets to send emails to other smtp domains using its services, that's what is called relaying.
This should explain why, in the above scenario, emails sent to internal users are received but not to external one. So for your application to be able to send external emails you should allow it (or its IP address) to relay emails through your exchange server and here is how to do it in Exchange 2007 and Exchange 2010.
A few days back I have talked about Installing your first Exchange 2010 Client Access Server when migrating from Exchange 2003 to Exchange 2010. Today I will discuss the requirements of installing your first Exchange 2010 CAS if migrating from Exchange 2007.
First we have to agree that that an Exchange 2010 CAS cannot communicate directly with an Exchange 2007 mailbox servers, so Exchange 2007 CAS are still required.
We will need to differentiate 2 scenarios when mixing Exchange 2007 & Exchange 2010 CAS
Exchange 2010 CAS and Exchange 2007 CAS in the same Active Directory Site
The first Exchange 2010 CAS server should be installed in your internet facing site, and it should be the first server people are directed to when they try to access your organization from the internet using, say, https://webmail.yourdomain.com/owa
Since Exchange 2010 Client Access Server cannot proxy the requests to an Exchange 2007 CAS server in the same active directory site, when the request reaches the 2010 CAS it will query Active Directory for the mailbox server of the user, if that mailbox is located on an Exchange 2007 mailbox server in the same Active Directory site the CAS will look for the ExternalURL & InternalURL parameter of that CAS server.
I am currently involved in planning and designing a couple of site resilient Microsoft Exchange 2010 clusters so I have been reviewing my notes and here is one I would like to share with you.
In short it is the summary of an 11 pages long technet document in 26 points I had previously highlighted on papers.