In the area of Tanzania where I live, mambo is a very informal greeting used by younger people or with people you know well. Jambo is more formal, can be used by everyone and is the more acceptable greeting. You would never say mambo to someone older than you or someone you do not know.
Some Tanzanians use the greeting Jambo when greeting foreigners. In most cases Jambo is considered to be a touristic greeting. Generally foreigners are associated with tourists and for that matter this greeting is commonly used with foreigners.
When I lived in Kenya, "mambo" was best described to me as translating to: "What's up?" This brings the understanding that it's an informal greeting generally used amongst peers or the younger generations. The response "poa" meaning "cool" would then be similar to the English response of "not much".
I have never heard this expression before. I learned some traditional KiSwahili conversation before going to Tanzania 12 years ago and the local people told me I had extremely good grammar and pronunciation. I was trying this as a refresher, but I feel like this Swahili is Kenyan or slang and not as proper. Not sure I want to continue unless these words are common and acceptable in Tanzania.
It's youth language - I'd use it with some friends as a kid in TZ (which also is 12 or more years ago). Sure, slang and Kenyan expressions are included, but this is a good start - it still is in beta, so there are mistakes. Also keep in mind that language changes and twelve years are quite long. Kind regards,