Otherwise, try sending a mass mailing to the whole class yourself so that you can be confident that there is something waiting for them in their electronic mailbox. Encourage your students to send a short reply immediately upon receiving a message even if they can't reply in full right away. The students on the other end are usually anxious to know if their message has arrived, too. Students should know the goals, type, and teacher's name of their penpal class. They should also know whether their penpals are being graded on this activity or not.
Students staring at computer monitors are functionally deaf.
Students really enjoy sending teachers email messages and getting messages back. Make macros of your terminal program to make the logging in and signing off as easy as possible. Give them screen shots of each step of the process.