Source: http://www.ego4u.com/en/business-english/communication/business-letter/salutation

Mrs – to address a married woman

Miss – to address an unmarried woman (rarely used now)

Ms – to address a woman whose marital status you don't know; also used to address an unmarried woman

Note: The abbreviations Mr, Mrs etc. are usually written without full stops (Mr) in British English and with full stops (Mr.) in American English.

Bon, je crois que je vais écrire Dear Debra :-)