Thank you for your post, and I apologize for the late reply.
I do not have access to a Outlook 2007 on a Windows 7 machine. My XP machine does work properly with FileMaker Pro 11 and Outlook 2007.
If you have discovered the cause or workaround, please post here, as a couple of other users have also reported the problem.
Anybody else having difficulty with Send Mail with FileMaker Pro 11 using Outlook 2007 under Windows 7?