This is an ongoing issue that has been reported by others. It appears to coincide with a Safari update, but not all agree on that.
See this link for more on the subject:
Thanks for the post.
As Phil suggested this is an issue we are aware of. As noted in the linked thread by TSGal the issue is from the Safari 5.0 per the Apple KB I have linked below. We have little information on whether it is Safari itself, the interaction with the Mail program or a glitch between the OS and applications caused by a Safari issue. The article does however list fixes to use as workarounds while Apple tries to resolve the issue.
Hope this helps.