SMTP call-ahead for recipient validation

We have two Cisco IronPort C360 appliances doing antispam and antivirus filtering before mail makes its way into Exchange. These have not been in a DMZ, and we're now making the effort to move these into a DMZ.

We use our corporate AD servers for LDAP recipient validation (so that invalid recipients are dropped). We were planning to move an LDAP server into the DMZ with the appliances, with email addresses pushed to it from production, for recipient validation. *BUT* I came across SMTP call-ahead, which seems like it would suit our needs better since we wouldn't need a dedicated LDAP server and our architecture would be more secure...

IronPort would open an SMTP session to Exchange to verify the recipient. Exchange hub servers don't do this out of the box, antispam agents need to be installed. This fellow has a good guide: This fellow has a good guide: http://www.jjclements.co.uk/2010/09/23/exchange-2010-recipient-filtering-on-a-hub-transport-server/

I'm wondering if what we're doing is a terrible idea. Does anyone have any experience with SMTP call-ahead? I would appreciate any thoughts, validation, or feedback.
msghydronAsked:
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Simon Butler (Sembee)Connect With a Mentor ConsultantCommented:
I enable recipient filtering on all servers that I deploy and it is something I think should be on all email servers.
The problem you will have is that Exchange has tarpit enabled by default, which can cause email delivery days. It will depend on whether the appliances are able to cache the results or not (I haven't used them myself, so don't know). If they are able to cache results then it shouldn't be a problem, as they will only have the tarpit delay the first time they connect. However if they are doing it "live" each time, then you may well have to reconfigure Exchange to remove the tarpit.

Simon.
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msghydronAuthor Commented:
We are proceeding with SMTP call-ahead on the IronPort C360 appliances, switching from LDAP recipient validation. We've removed the 5-second tarpit delay. I think this is a better configuration, when the C360's are in a DMZ.
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