MacBookPro OSX 10.8.5
Mail Version 6.6 (1510)
If I try to create a new email by typing all appears to be normal.
But from time to time a block of text is suddenly surrounded by a rectangular line frame, with a X at one corner.
Clicking on the X makes the enclosed text disappear.
Clicking somewhere outside the frame makes the frame disappear.
It may be associated with a chunk of text pasted in from elsewhere, though I cannot prove that.
What is going on?
How can I disable this confusing "enhancement"?