See the safari post below. It is the same problem that you are having.
FileMaker should work with Apple for a solution. Just pointing to an Apple comment that has a manual effort to fix the problem is of no help for sending thousands of clients information automatically. Will FileMaker own this problem and work with Apple for a solution or is this just how it is going to work (that is, NOT work) going forward?
Yes, we are working with Apple on a solution. Every third-party application that uses the Send Mail API provided by Apple is having this issue. Although we expect Apple to fix this, we are evaluating ways to work around this problem. As it turns out, Safari 5.0.1 was just released and it appears to have fixed this problem.