recipient name is not recognized

I’m using Outlook 2000 to send a mail to a person and I received an undeliverable mail from the system administrator. It states:

The recipient could not be reached:
elaine.ho@st.com
The recipient name is not recognized.

I send a mail using Yahoo mail and it seems fine, I did not receive any undeliverable mail.

Is this problem on my side or on receiver side?

thanks and regards,