Same thing happening here, and it is always in conjunction with html in the email. Specifically, it has been with web content copied and pasted into an email. I've had to instruct the users to "paste special" and paste text only.
Our experience has been that the client freezes with clicking the "Send" button right when the spell check should appear. The spell check interface never does appear, and you have to forcibly terminate the client (ver 8.5.1 with 8.5.1 mail template).
Feedback response number WEBB84KP82 created by Mike Gotts on 04/16/2010