It sounds like Outlook isn't properly handling the Sendmail function. Or it could be that FileMaker is sending it too fast or something like that. If I really needed it to work, I would probably reinstall Outlook and FileMaker and start afresh. Do they need to preview the email first? If not, send it directly to the SMTP server and skip Outlook.
I've found the FileMaker send mail script does not work well with Outlook on the Mac. I was forced to write an AppleScript to work around it.
FileMaker does not work as seemlessly with Outlook as it does most other mail programs. Likewise, Outlook does not play particularly well with the Mac OS X Server mail program either even though it can be made to work. Outlook realy is designed to work with an Exchange server and interacting smoothly with non-Microsoft software is not as high a priority for Microsoft and it shows. Basically, I tell clients that have a Mircrosoft Exchange Server, then use Outlook. If you use a different server, then look at other mail application options.
Keep in mind that FileMaker has only rudimentary email support. But some of the plugins give it much more functionality. If you really use email a lot in your solution, you might want to look at some of these plugins. I've run into times when I couldn't get Outlook to work with FileMaker natively, but with plugin functions, it worked fine.
looks like there seems to be a common theme with Microsoft product, consistently inconsistent.
Found an interesting one yesterday, FM 12 (letest rev) on Windows, a layout that had two container fields, one on top of the other (the top field being a calc representatiion, the other being the source field). On one PC, pdf's would show up perfectly, on another some records would show the field underneath (being the source field), I went out of my head trying to work out why. Ended up just moving the fields so they don't overlap. Weirdness.