launch URL with mailto yields inconsistent 'can't open file' error

[Mark E. Powell]

Could use some wisdom and a Scotch on this one.

I am having problems with launch URL failing as the content string gets long.  In my tests, when the total mailto argument resolves to between 1645 and 1980 characters, the launch fails with 'can't open file'.  The character count varies, and no discernible pattern to anything that I can see.  Any ideas?  Is it an OS thing?

I am on Windows XP, Rev 2.9.0.

Mark