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You should not start new threads for an existing issue as it just spreads the information around and those trying to help you may not be aware of previous answers given, and therefore waste time by duplicating their efforts to assist.
Note that the most recent posts are brought to the top of the list.
Your question has been answered, with more than one way of resolving the issue.
It is not a DNS issue so do not add or change DNS settings.
The issue is that you want to split the hosting for a domain to different machines, web on one, mail on another and sme server needs some instructions re how to deal with that.
Personally I would check the settings for DNS for that hosted domain, in Domains panel on sme server, and set DNS to Internet DNS servers, rather than Resolve locally. This sends all local requests for that domain to external DNS servers, which will resolve www requests back to your sme server, and maybe just (meaning I'm not sure) resolve mail requests to the SBS server via the external MX records (as you already have configured those correctly). If already set to resolve to Internet DNS servers, then read on.
The "ultimate answer" for you is to "delegate mail" to another server, as already advised.
It is very easy to delegate mail to another existing mail server, everything else will stay the same & should take under 5 minutes to do from log in time to log off time.
From
http://wiki.contribs.org/Email#Deliver_email_for_one_domain_to_an_internal_mail_serverdb domains setprop hosteddomain.com MailServer SBSserverexternalIP
signal-event email-update
replace hosteddomain.com with your customers hosted domain name
and replace
SBSserverexternalIP with the fixed public external IP of the SBS server ie 123.456.78.90 (or whatever)
An alternative approach is to NOT configure the "whole" hosted domain on SME server.
Only configure the www part ie
www.hosteddomain.com and NOT hosteddomain.com
Do this in the Domains panel, and you can check entries in the Hostnames and Addresses panel
As I interpret Charlies advice, this means that
http://www.hosteddomain.com/ will resolve to the hosted web site on sme server, but
http://hosteddomain.com/ will not resolve, but you can provide local workarounds (redirects) for that if preferred.
So mail sent to hosteddomain.com will follow the external MX records and be delivered to the SBS server, as that domain name does not exist on your sme server.
I think it's simpler just to setup mail delegation for that one domain in question.