L'adresse is feminine gender but begins with a vowel. The posessive pronoun in this case has to be in the masculine form hence mon adresse not ma adresse which you can see would be difficult to say with two vowels together. It is similar to the elision that occurs when a vowel starts a word - l'adresse not la adresse.

That is why, when you learn a new common noun in French, it is very important to learn it with its article. Le stylo => (m/t/s)on stylo La feuille => (m/t/s)a feuille. Bravo, continuez ! José