Capture outgoing email

[Andy Beverley]

Hi,

Is there any way that I can capture the exact content of an email as it 
is delivered by Postfix to an external host? I am looking for something 
different to always_bcc or similar, in that I want to capture the exact 
email content rather than deliver a copy of it elsewhere.

The reason is that a recipient host is claiming that DKIM signatures are 
broken from my server, despite them being correct for all other 
recipients. I would like to take an exact copy of the email as it is 
delivered, and run it through an independent DKIM checker (opendkim is 
used as a milter to sign the emails).

Thanks,

Andy