Hi,
What the documentation means is that on Windows Mail you can modify
and commit changes only from DLL plugins and while messages are not
currently selected, this is because of an internal Windows Mail
restriction.
Are you making an EXE or a DLL plugin?
Also, before modifying a message you can check if it's currently
selected, and deselect it with
OEAPIObj::SetSelectedMessageID(-1)
in that case.