Nathan may be referring to a regional use.  My experience matches the dictionary definition.

Ms. (miz) - modern, all-purpose title for a woman

[New Oxford American Dictionary]

Ms. | mɪz, məz | 

noun a title used before the surname or full name of any woman regardless of her marital status (a neutral alternative to Mrs. or Miss):

I am a native speaker of American English, so this may not apply to other dialects, but:

In the US we use Mrs. and Ms. (Misses and Miss respectively). 

Mrs. refers to a woman who is married (ex: Mrs. Smith)

Ms. (pronounced Miss) refers to a woman who has never been married (ex: Ms. Smith)

Ms. (pronounced Miz) refers to a woman who has been divorced and not remarried (ex: Ms. Smith)

Normally, however, in my region we just use Ms. regardless of marital status. The whole concept is pretty outdated in my opinion. 

I assume that Mdm. in your question is Madam, which is not used in the US, but in Britain I believe. I hope this helped!