Exchange 2010: Send emails via STMP with custom From address to outside the domain

Question

The requirement(s):

(1) Connect to Exchange via STMP and (2) basic authentication and send emails with a (3) custom From address to (4) recipients outside the domain.

I was able to get (1) - (3) working. I created a dedicated receive connector for this task and configured it like this:

Permissions:

• ms-Exch-SMTP-Accept-Any-Recipient (for authenticated users)
• ms-Exch-SMTP-Accept-Authoritative-Domain-Sender (for authenticated users)
• ms-Exch-SMTP-Accept-Any-Sender (for authenticated users)

Authentication:

• TLS
• Basic Authentication (without TLS)
• Exchange Server Authentication

However, I'm still struggeling with (4): I can send with "fake" From addresses to recipients inside the domain. Also, I can send with the original From address to recipients outside the domain.

Can you tell me what I'm missing, to configure Exchange to send emails with changed From addresses to recipients outside the domain? (Or is this even possible at all?) Thanks.

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UPDATE

I have to correct myself: it seems to be working after all. There must be some issue with the mailbox I used for testing. It turned out it's working with other external mailboxes. However, I still have no idea what was different there... Anyways, you can take this as a documentation on how to configure Exchange in such a way ;)

Answer 1

What kind of error do you get? Relay not allowed?

Generally, as far as I know, Exchange 2010 doesn't allow relaying mails (i.e. accept recipients that the server is not authoritative for). That's probably, why you can send mails to internal recipients. In fact, you should be careful not to block legitimate mail from outside your organization to enter the server.

Depending on your infrastructure, normally Exchange will accept all mails via SMTP (i.e. port 25/tcp, but not 587/tcp) from outside (probably after spam filters) to recipients it is the destination for, even if the sender is not authenticated. Otherwise no one could send you an email. Of course, if you have another MTA on your network border which accepts mails from the outside and then relays it to the Exchange server, the situation is different.

I don't have an Exchange 2010 test server handy right now, so I can't test my statements. But off the top of my head I would say you'll need to check the "Externally Secured" in the authentication tab of the receive connector to be allowed to relay. But as far as I know, enabling this deactivates all security measures, i.e. authentication, so you should be really, really careful with that or you could end up with an open relay!

Also, may I ask what's the purpose of your scenario? From where do you connect? Is step (1) from inside or outside your network? If you just want to be able to send mails from another server from inside your organization/network to an outside recipient, the easiest way would probably be to enable the above-named "Externally Secured" and restrict the IP source of the sender to your server's IP in the Network tab.