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SMTP setup question

Ok, I have a quick setup question regarding SMTP smarthosts and Exchange 5.5/2003.  I still have a mixed move Exchange site with both Exchange 5.5 and 2003.  I use a single Exchange 5.5 server with the IMC for all outbound email.  Even Exchange 2003 routes mail to this Exchange 5.5 server for outbound messages.  Here is what I want to setup, so someone check my plans and see if you can find anything wrong with it..

1. Setup new SMTP server in front of our SPAM appliance, inbound mail will be accepted for valid domains only and checked against an LDAP lookup.  Then forwarded on to our SPAM appliance.  Messages will get scanned for SPAM and then forwarded to our main Exchange 5.5 server with the IMC installed.  

2. Configured this same Exchange 5.5 server with the IMC, to use a smarthost and route all outgoing mail (SMTP external messages) and use this same new SMTP server as a smarthost.  

My first question would be, if I do this, will the new SMTP Server I setup be able to relay outbound mail and accept inbound mail?  If its configured to relay all mail to my Exchange 5.5 server, won't trying to send outbound mail through it as well, re-route mail back to my Exchange 5.5 server?  

What I'm trying to accomplish is a reduction of backscatter and misdirected NDRs.  I need more control of NDRs than Exchange 5.5 gives me.  I am not yet ready to migrate to Exchange 2003 for our inbound and outbound mail systems yet.  This setup should also reduce problems with our spam system as it currently gets overloaded and there are known issues with their SMTP service.  So fielding mail with a new SMTP service on a different machine should relieve some of our SPAM system problems.  A side issue we are having right now, is that we are getting blacklisted on various blacklists due to spoofing and mis-directed NDRs.  I need to better control inbound mail which will reduce NDRs from the inbound side, but also use a smarthost to control how outbound mail is sent and over which server and IP range.  Any suggestions are welcome.

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8/22/2022 - Mon

will for the first question u will configure the smart host to accept relay only from specific hosts
for question 2 and for a full detailed explanation check this page out

That partly answers my question.  But again, if I setup an IIS SMTP Server for incoming mail, setup my remote domains and have it forward mail to my actual mail server, what if I want to use the same SMTP server for outgoing mail?  If the SMTP server is configured to relay all mail through a smarthost basically, which would point to my internal Exchange 5.5 box for incoming mail, won't sending outgoing mail through that SMTP server just get caught in a loop?  For example, the SMTP server would be setup for incoming mail, accept messages for domains I specify, and then relay to my internal exchange 5.5 box.  Now when I send outgoing mail through it, won't it just relay the mail back to the exchange 5.5 box and get stuck in a loop?  Will I need to setup 2 SMTP servers?  


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