Why does IE not allow the selection of a default mail handler?
In this case the person asking also reported that the actual drop-down
selection box was blank.
If clicking the little down arrow at the right of the selection box results
in an empty list, then some configuration information is missing from the
system to let it know the available programs that could fill the role as
default email client.
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I’d first make sure that you were logged in as an administrator since
that’s required to make this change. But assuming you are, I’d run a system
file check to see if something’s broken and possibly even reinstall IE6 to
force it to rebuild the settings.