Sometimes (okay, a lot of times) I forget that not everyone is as familiar with letter writing as I am and I thought it'd be a good time to go over a refresher for addressing a letter. Now, you know, I like to get all fancy with it, but the basic rule is the TO address should be in the middle of the front of the envelope with the ZIP code in the lower right-hand corner. This is to make it easy for the automated postal system to scan and route the letter for proper delivery. The ZIP code is what lets that happen.
The machines are programmed to scan for the delivery ZIP code starting from the bottom right-hand corner and moving across the letter until it finds it, reads it, and processes it accordingly. This is why, in the U.S., you don't want to write your return address on the back of the envelope. There's a good chance that the letter could end up fed backwards into the scanner. Since your ZIP will be the first one scanned, that's where the letter will then deliver to. It's good to remember that you are dealing mostly with machines here, not people. A human would understand that you mean that's the return address, but the APC doesn't care.
You should ALWAYS ALWAYS ALWAYS put a return address in the upper left-hand corner. I use a return address with my mom for gosh sakes. It's good to do because then the respondent doesn't have to go hunting down your address. This is especially important for new pen pal relationships. For your initial pen pal letter, you should write the address both on the envelope and inside your introductory letter. This solves many problems.
For example, if you have really expressive handwriting, it can be easier for your pen pal to compare the two if they are trying to figure out if you wrote, say, a 2 or a 7. If the recipient loses your letter or your envelope, they will still have your address. I still feel bad for not being able to write back Marco from Italy from back when I put my address in SCRYE magazine in high school. I didn't mean to not write back. I just lost the address. Don't be like me and Marco.
Oh, and make a habit of keeping an address book! However, just because your pal SHOULD have your address, don't rely on that. Like I said, I send my own mom my address (and she constantly texts me for it anyways) when I write to her. This is especially important on postcards. If you want a response, include your address. It's that simple. Kathy and I designed a few custom return address stamps to be purposefully small because we wanted to be able to use them on postcards.