"qmail-send program" - Help Please

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:

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

THANKS