>Just so I'm clear in my own mind; adding the Source="From" to every print directive means [...]
In a 'normal' message, the sender address (=mailbox owner address) is specified in the 'From'-header (there's no 'Sender'-header in this case.)
When the message is sent on-behalf, then the mailbox owner address is specified in the 'From'-header and the 'real' sender (=delegate) address is specified in the 'Sender'-header.
The default behaviour of a print directive (when the "Source" parameter is not specified) is to use the 'Sender'-header when it exists and otherwise to use the 'From'-header.
This means that by specifying the parameter 'Source="From"' you force the use of the mailbox owner address for AD-lookups.
-Franz