Anonymous“Simply reply To Whom it May Concern:
”
No, no, and no!
You know who sent the correspondence. You know her name. The only thing you don't know is whether to use "Miss" or "Mrs." or "Ms." As this thread should have made abudantely clear, if you don't know, use "Ms."
You use "To whom is may concern" only when any number of yet unkonwn people may read it - a letter of recommendation, perhaps. You would never use it when you know the recipient but simply not her marital status.
It's time to lock this thread before any additional well-meaning but totally erroneous advice is given.