Wow! There are a lot of places along an email's route where it can be "lost in outer space." When you see how an email must travel, it makes you wonder how any email gets properly delivered.
With this email flow in mind, here are some things to check if someone isn't getting your emails:
1) If your friend uses an email client, ask him or her to make sure your email isn't on their client's black list.
2) Ask your friend to log into his or her account on the web and check the Spam filter. If your email provider allows it, he or she could disable spam filtering on their account and then see if they can receive your email.
3) Try emailing them from a different email address. (If you don't have a second email address, you could temporarily set one up.)
4) Ask them to send you an email and you reply to it. If they receive your reply, then things may be working again.
5) Call your email provider and have your friend call his or her provider, as well. Ask if there are any known issues between servers.
That's about it for our answer. As I stated before, there are no easy answers! Sometimes, emails are just lost for no reason and that's frustrating!