Oops. I forgot you like periods with abbreviations in US English.It’s Dr. with a period in US English.
It occurs to me that Mrs. only works if she’s married.
Since you cannot use "Dear Dr. X," then "Dear Dean X" is the best choice.Thank you for all your answers. Indeed I am not sure if she has a Doctorate but I am sure that she is married. So I will go with "Dear Dean X"!
Maybe, or maybe not. It might well have that effect. On the other hand, it might be taken as evidence that the writer hasn't done his/her homework.If I don't know whether she holds a doctorate, it is not a bad strategy to flatter her by assuming she has one.