Why can't someone receive an email from me but I can receive their email?

why can't someone receive an email from me but i can receive their email?
Are you using a free web-based email like Yahoo! Gmail, or Hotmail?

May be their side block your email or filtered your email as spam mail.

Do they use free web base email like Yahoo, gmail or msn ? IF they use , ask them to check on the junk mail folder.

Regards,
ABB

Try sending an email to yourself. That may help determine if the problem is on your end.