It is important to keep a list of who is who's keypal. I create a web page for my students to use and a poster that is hung in the classroom. I make a printed copy of each email message. Eventually this goes into my student's writing portfolio's.
Students keep electronic copies of their email messages also.
This helps in case we loose a letter or need to send it again. Don't keep the students waiting too long. Sending an email once a month is a good goal. That means in one month your students will receive and send a letter. Twice a month is even better but give yourself time to get used to the process.