Sending and Receiving email from an Exchange organization but behind another orgs. firewall?

tenover used Ask the Experts™
I have a problem here....Part of our company was purchased and is undergoing a "transition" for a few months.  These 20 users all run Outlook 2007, are sitting on my LAN and use my Exchange server.  The company that purchased this part of our business already has set up email accounts on their Exchange server and have told the users they can use OWA to access it, but the users are not happy about this.  Is it possible to still sit behind our firewall, be on our domain BUT connect Outlook to this other companies Exchange server and be able to send and receive email from their new accounts?  I'm thinking there wouldn't be a problem connecting and receiving email, but there would most likely be an issue when trying to SEND email from behind our firewall......I don't want to open up relaying on my Exchange server....Thoughts?  Advice?  Thanks.
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Does the other company have Outlook Anyhwere (RPC over HTTP(S)) enabled?

The users can create another profile in Outlook, and configure Outlook Anywhere.

 Also, they could establish a VPN connection directly to the other company's network using a VPN client, and then use their alternate profile of Outlook.


I know how to setup both of those options, regardless of which one they want to use, but would these clients, who are still on OUR domain, have any issues sending and receiving email from these new email addresses?
No because if they are using Outlook Anywhere, or a VPN, they are establishing a secure tunnel to thei other network.  Even if you block outgoing port 25 in your network, the traffic between their Outlook client and the remote Exchange server is channelled through the secure tunnel.

Outlook Anywhere uses an SSL encryption
VPN should use IPSEC

As long as they CAN establish the connection, send/receive is no problem.


Gotcha, thanks.  And I DO have port 25 blocked on my firewall with the exception of my mail server.....
Most excellent.  This should be done everywhere.  Nothing worse that someone's virus machine affecting the entire organization's ability to send mail.

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