Actually, the options I included in my question were the ones I tried out last night, so they were not "without success" at the time of writing.
I had tried other options without success the previous days, but this morning I received an email reply from the users of the relevant domain that...
Hi, I have a Linux VPS which serves 20 some domains and Plesk Onyx to manage it. I have set up DKIM on my VPS which works ok for all but one, which uses an SBS2011 server with Exchange 2010.
They have a problem because Exchange sends emails directly via a smarthost and the IPs of the receiving...