I think that the word you want is probably "correspondent," or if you want to be less formal, maybe "pen pal" (especially if you are writing to someone from another country). These are people who write messages. A "delivery girl" would not be someone who writes messages, it would be someone who carries packages from one place to another and "delivers" them to someone. Except that these days, it would be more respectful to say a "delivery woman" or a "delivery person." "Messenger girl" is not a phrase that we use in English. A messenger is not someone who sends a message, but is someone who delivers a message. "Because I could not wait for the mail (Brit. post), I hired a messenger to carry the letter to my friend." A delivery person is usually delivering a package, whereas a messenger is usually delivering information.