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It sounds like your email address may be blocked or ending up in a spam or junk folder on your friend's computer. I'd suggest your friend do the following:
1) Check their spam or junk folder first
2) Check their mail rules to make sure your email address hasn't been added to a "blocked" list
3) Search their computer (use Spotlight on a Mac) to see if your email address appears in any folders (such as a firewall or security application)
4) Contact their email service provider for further instructions
Here is some general information regarding troubleshooting sending and receiving email:
I hope this information helps ....