I recently had an incident where a client running Exchange 2007 could not send mail to his chosen smarthost. In this instance the customer did not want to use DNS to route outbound internet mail but rather through their ISP - using the free and not premium service.
Exchange could not send email through the smarthost, however using that smarthost in an email client allowed mail to pass without any authentication.
It turned out that the smarthost didn't support pipelining (see here) so the Send Connector was modified to send out via SMTP instead of EMSTP.
This simply means Exchange sent a HELO rather than an EHLO.
To modify a Send Connector to send via HELO perform these steps in Powershell.
Set-SendConnector -Identity [Send Connector Here] -ForceHELO $true
More information here
More information here
Oliver Moazzezi
MVP - Exchange Server
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