LR3.5, Windows 7 64bit, Outlook 2010. After selecting several photos in LR3 to be exported as JPEGs to Outlook, the latter shows but one photo as an attachment. Trying to export those same photos again, I'm greeted with the following error, "Problem Exporting Files". The error message further states that the files already exist. I've had to send each photo separately to get around this hurdle.
What I usually do is create a folder just for storing emails I wan to send. I have LR export all the images to that folder, then in Outlook, I select all the images to once as attachments. If I recall Outlook had an issue will dealing with attachments from another app. Simple mime I think. Outlook express works fine but outlook 2007 had that issue, so thats my work around.
It sounds like you might be using the email preset that comes with the LR software. It has always been a bit problematic for me in Windows. I suggest the following plugin which has worked for me. There are others in the exchange as well.
Europe, Middle East and Africa