a) Setup internal Ex55 to only accept connections from DMZ Ex55 (external)
- internet mail service
-- connections
--- specify by host (add DMZ Ex55)
b) Setup internal Ex55 to forward all mail to DMZ Ex55 (DMZ Ex55 acting as a forwarder)
-internet mail service
-- connections
--- forward all messages to host
c) Make sure that DMZ Ex55 is not configured as an open relay:
1) Open the Exchange Administrator program.
2) Expand the site that contains the Internet Mail connector.
3) Expand "Configuration."
4) Open "Connections."
5) Open the Internet Mail Service.
6) Select the "Routing" tab.
7) Select the "Reroute incoming SMTP mail (required for POP3/IMAP4 support)" radio button.
8) Press "Add?" to enter your primary mail domain and any domains that you host mail for.
9) Configure the domain to "Should be accepted as inbound" and click OK.
10) Press the "Routing Restrictions?"
Select the "Hosts and clients with these IP addresses" check box but not specify any IP addresses and click OK.
11) Click OK to close the IMS properties dialog.
12) Stop the IMS.
13) Start the IMS.
Otherwise i'd take a look at virus protection as well (at Internal Ex55 level) and applying and relevant security patches (OWA if being used etc).
hope this helps,
regards,
jb.
