"And so you absolutely opened what was directed to her! Oh! Mr. Weston (laughing affectedly) I must protest against that. A most dangerous precedent indeed! I beg you will not let your neighbours follow your example. Upon my word, if this is what I am to expect, we married women must begin to exert ourselves! Oh! Mr. Weston, I could not have believed it of you!" Chap 36

Mrs.Elton's reaction to Mr.Weston reading a letter addressed to his wife seems so severe that I have invented a theory (worthy of Emma), to explain it. What if Mrs.Elton has a gulty secret (or two) and is afraid of her past catching up with her in the form of a letter? Perhaps it is a rejected suitor who vowed revenge. Maybe that was why she was so quick to marry Mr.Elton in such a short a time-period. Hmmm...