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I assume your ordinary mail flow is okay? ie: you can send and receive mail? My guess would be that this is an attempted mail relay.. ie: someone is trying to send an email using your server. If you do a reverse lookup on that IP address you get: 247-228-112-92.pool.ukrtel<wbr />.net. Is this familiar?<br />
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By the looks of it your server is protected against open relay but you might just want to check at <a href="http://www.mxtoolbox.com" rel="ugc">http://www.mxtoolbox.com</a>&nbsp;..<br />

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The normal mail works fine. &nbsp;And the reverse lookup on that address means nothing to me. &nbsp;It's not related to us.
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Okay, run the open relay check.. <br />
You can use mxtoolbox.com to do this or run a check yourself using this article: <a href="http://www.amset.info/exchange/smtp-openrelay.asp" rel="ugc">http://www.amset.info/exchange/smtp-openrelay.asp</a><br />
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You'll probably find that your mail server is secure and these messages are simply an indication of an attempt to relay through your server. This error is generated when there is no record of the domain in the Recipient Policies.
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There is no open relay (found by following the above link's instructions.) &nbsp;Do you think that these are just attempts to use this as an open relay? 
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This is an SMTP protocol log for virtual server ID 1, connection #759. The client at &quot;92.112.228.247&quot; sent a &quot;rcpt&quot; command, and the SMTP server responded with &quot;550 5.7.1 Unable to relay for ben@applecentrecentralcoas<wbr />t.com.au &quot;. The full command sent was &quot;rcpt To:&lt;ben@applecentrecentral<wbr />coast.com.<wbr />au&gt;&quot;. This will probably cause the connection to fail. For more information, click <a href="http://www.microsoft.com/contentredirect.asp" rel="ugc">http://www.microsoft.com/contentredirect.asp</a>.<br />
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This is what appears in the event log. &nbsp;There are about 40-50 per day, the ip address means nothing to me and the domain name means nothing either, it's not related to us.<br />
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Any thoughts
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This is an SMTP protocol log for virtual server ID 1, connection #759. The client at "92.112.228.247" sent a "rcpt" command, and the SMTP server responded with "550 5.7.1 Unable to relay for ben@applecentrecentralcoast.com.au ". The full command sent was "rcpt To:<ben@applecentrecentralcoast.com.au>". This will probably cause the connection to fail. For more information, click http://www.microsoft.com/contentredirect.asp.

This is what appears in the event log.  There are about 40-50 per day, the ip address means nothing to me and the domain name means nothing either, it's not related to us.

Any thoughts

Each day I get about 50 events like this one. Unable to relay for ....@..... 550.5.7.1

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

Exchange

Small Business Server (SBS) is a line of server operating systems targeted at small businesses by bundling the operating system with a number of other Microsoft products that would normally need to be purchased or licensed separately. The most notable inclusions are Exchange, SQL Server, SharePoint and ISA/TMG (Microsoft's firewall and proxy server).