I have unlimited calling and texting on my phone. However, I have 500mb of data, which i suppose that this is monthly or annually (I don't know so I use my home's wifi network).

So when I subscribe to receive emails on my phone, it said that "data rate may apply". I also turned of my mobile date so that I won't waste my 500 mb, so i rely on my wifi to use it for apps and social networking...

what part of "may" are you confused about - go look it up in a dictionairy.