I am having a problem using Internet Explorer 8 (IE8) to open mailto links with long messages.
After the user clicks on the link, IE changes to an about:blank page
it appears that the following hyperlink seems to work. It is 512 characters long and seems to work every time. You can increase the length of the To, but you must subtract either from subject or body. Same with subject.
I wish this was documented somewhere; I am just guessing from experimenting with this.
I will gladly accept any answer from a person that can track down documentation that indicates that this is a known issue.