Language usage with a Japanese parent

Hello everyone,
I have a friend in Japan (the same age as me, 18 years old). When I went to Japan earlier this year, I saw her on three occasions. The third was the most memorable, as I got to visit her town in 埼玉県 and meet her family. I had a great time! They even prepared a whole 餅つき experience! I felt it was too much, but regardless it brings back good memories. Her parents were very kind and enjoyed meeting me.
Now back in the US, I still communicate via email with the girl. Recently, however I received an email from her mother. She said that she wanted to exchange mail with me as well, which I obviously couldn't deny.
Now, here's where my question comes in. How should I address myself in the email? Should I use 私、or is 僕 okay？I speak mostly in ます form, which is what she has replied back in. Should I use 謙譲語? I don't want to create a bad image for myself or sound rude because I am only used to communicating with people around my age. If anyone has some good input on this topic, I would greatly appreciate your assistance.
Thank you.

Jeemusu,

For an 18 year old, "boku" would be the natural term to use to represent yourself.

Using kenjogo to a friend's mother is absolutely normal and often expected. You do realise that keigo consists of kenjogo, sonkeigo and teineigo.

Generally speaking, you're safe as long as you use language that you would use to your language teacher.

Well, I sent an email to hear using the knowledge I had to keigo. While she obviously praised my usages, she specifically stated that I used「御座います」too much. She even said that she felt the response was too polite!

Ugh, I guess I still haven't figured it all out yet. So when you use keigo, it seems that you shouldn't use it at every single instance. For example, every time you would say 「です」, you would change it to 「で御座います」. So, you use both together within the same piece of writing?

I understand the grammar rules that need to be made to the writing, but I suppose I need a little bit of guidance as to how to use them properly.

I think you're getting way too hung up on keigo. If you use the desu/masu form correctly and clearly, the recipient will be more impressed and appreciative than if you sort of stumble around trying to use keigo. (For example, de gozaimasu is really not used very much, and if you don't know when to use it, just don't!)

A great many Japanese people don't know how to use keigo properly (or at least, they worry a lot about making mistakes). They do NOT expect foreigners who are just learning the language to use it flawlessly. Believe me, you are not going to offend anyone if you fail to use honorifics. Better to get your ideas across with the desu/masu forms, which are still considered polite.

Jeemusu wrote;
"she specifically stated that I used「御座います」too much. She even said that she felt the response was too polite!"

I agree with Uma. This is like using "mam" a lot and then being said, "Oh honey, you're being too polite. Just call me Mary."

"gozaimasu" is perfectly unoffensive. Hey, if I write too much "desu" on a letter to one of my junior high school teachers, she'll write back saying I should use "gozaimasu". I went to a strict Christian school.

There's probably no problem with your usages. Just be flexible and adjust to what the other party requests, as you would in any other language.