"qmail-send program" - Help Please

[HelixOmnimedia]

Hello,

I'm trying to send an e-mail to a friend in the USA - just a plain text e-mail containing my new e-mail address, home address and telephone number, and asking a few things too - obviously
I've tried to send it twice, once this morning and another a few minutes ago, but I keep receiving this e-mail back:

Hi. This is the qmail-send program at yahoo.com.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.

<NAME@comcast.com>:
24.40.8.248 does not like recipient.
Remote host said: 550 Mailbox unavailable or access denied - <NAME@comcast.com>
Giving up on 24.40.8.248.

--- Below this line is a copy of the message.

What is causing this happen? Is it at my end or their end? And how do I fix it?

THANKS