To be sure, did you first click on Tools > Options > Emails, and next to "Default Letter Style", select the option "Use Default" ?
If so, and the problem continues, please try resetting the Internet Explorer settings as instructed in the online article at:http://support.microsoft.com/kb/923737
After that, please restart your computer to ensure the changes take effect.
If the problem still continues, I would like you to create a new identity in IncrediMail and see if you still encounter the same problem.
To create a new identity, please click on Tools > Identities > New Identity.
Set up your identity and see if the problem continues to occur in the new identity.
Please note that I would like you to configure the same email address in your new identity, there is no need to create a new email address. While the identity name should be different, the email address should be exactly the same.
As everything is being done in the new identity, none of the data and settings in your original identity should be affected.
To switch back to your original identity, simply click on Tools > Switch Identity, select the appropriate identity, and click on OK.
Please let me know the results.