Is it Incredimail or Outlook? Those two are not the same.
This is essentially an Incredimail issue. Your preferred email client
should be designated as the default. To check this:
Open the Default Programs applet, which you can access either from
the Start menu or via the Control Panel, then click the first item:
"Set your default programs."
After a few seconds, a list of programs comes up. Click on "Incredimail".
If it doesn't respond with "This program has all its defaults"
then fix it by clicking on the option indicated by the first green arrow.
If Incredimail continues to fail, consult an Incredimail users forum.
