STD Mailing question

If a person is married, but chose to keep their last name, I know I still address it to to both, but do i have the lady listed as

Ms. (maiden name)
or Mrs. (maiden name)?

I have no idea how you would word that, i actually dont think there is a wrong way. Either one would work.

** I also wanted to add that someone told me not to drop the enevelopes with magnets into the post office mailboxes, actually bring them into the post office. They stuck them in a box and they all stuck to the sides of the mailbox. (cause its metal) I wouldnt have though about that! Just thought i would share!!