The following is a very good example of a letter from a man to a woman! I myself would definitely reply!

Dear Masha! (Very important is to start your letter with Dear, Beloved or My Precious or whatever salutation feels appropriate for the depth of your relationship). Thank you for your letter! You have filled my day with happiness and joy! I found myself calm and comfortable. (Be sure to thank her if you're responding to a letter, and mention the number of times you've reread it).

My love for you is growing stronger and stronger every day and every letter. We can probably agree that this is very early to grow feelings to each other, but still, I do have a strong feeling for you and I gladly allow me to have it!  You must be very special. I believe you are. 

I am honored that you chose to write to me among thousands of men. Thank you for opening the door of your life to me. I hope I will not disappoint you. (Describe how your loved one makes you feel. Try to be original, but put sincerity ahead of creativity).

I would like to call you soon. Then we can exchange email addresses and make it possible to send photos of each other.

Like you, I am looking for my second half in this world. A loving wife, a best friend, a partner and confidant, I will love her with all my heart and I hope it is you. I wish you were here right now, so we could go for dinner and keep the conversation going, one-to-one. (It is good sometimes to describe your hopes for the future).

Maybe I am a little mistaken, but I feel we have a wonderful opportunity to get closer to each other and find out if we are eager to share our feelings. Time gives us a chance to grow an intimate relationship based on love, understanding, honesty, faithfulness and commitment.

I will close for now and hope we can get in touch again soon. I am really excited about the possibility of you and me making a successful couple. I will certainly do my best to make everything work out right!
(Close with an exhortation to write back quickly, a mention of the next time you expect to see each other, or another appropriate comment).

Sincerely, John