Your changes could not be saved because you don't have permission

[Guest]

User receives HTML formatted email then does the following:
opens email
clicks on Edit>Edit Messagae
clicks on Format>Plain Text
clicks yes to warning message about format changing
clicks on Edit>Edit Messagae
clicks on Format>Rich Text
Adds text to the body of the email (text is font color 'red')
User then tries to save the email but receives the error:

"Your changes could not be saved because you don't have permission to modify
some or all of the items in this folder. Do you want to save a copy of this
item in the default folder for the item?"

User is unable to save changes. I try the same process and experience the
same result.

Any ideas on what might cause this?