I send the email; the recipient tries to reply to the email and it doesn't send for them. I am not sure if it's somewhere in my settings that I have typed an error in the email address to reply to. I am using Outlook Express on a MacBook Air.
Ask them if the address they have replied to has the correct spelling. If you type the wrong address in the reply to field when setting up your email the emails will send but the address will be wrong when they hit reply.
@Theresa2553 Email addresses get stored in cache and if at some time the wrong address has been entered the program will keep entering the incorrect address even if you have the correct address in you contacts.
Have a google on how to delete cached email addresses from the email applications you are using.