I've been having exactly the same problem when sending with Office 2007 under Windows 7 using Photo Mechanic.
However, I think I have cracked it... though am not sure how or if I can instruct Photo Mechanic to get round the problem, but am fairly sure it is bug of some description.
Unfortunately the only way round it at the moment (that I can find) is to tick the "Review email in email application before sending" box and do the following manually:
When the email pops up with image attached double click any email address filled in the "To...", "Cc..." and "Bcc..." boxes respectively, a box will appear with the title "Email properties". On the right of the "Email type" bit, select "Internet Type". You will see the box to the left greyed out with SMTP filled in.
Then just send as usually. Unfortunately the above needs to be done for every email you are sending with Photo Mechanic.
Oddly enough when sending with Photo Mechanic the "Email type" box has the email address I'm sending to in it eg. "Email type: EMAIL@EMAIL.COM".
If the guys at Photo Mechanic could look into this it would be greatly appreciated. Maybe we've all got our settings wrong somewhere. However, my laptop is a relatively fresh install and is up to date, and I have tinkered only minimally with Outlook and not enough to knowingly upset and settings.