What is the difference between "No address associated with name" and "Unknown host"

[a at zeos.net]

What is the difference between "No address associated with name" 
and "Unknown host":

$ ping accounts.eirtrade.ie
ping: cannot resolve accounts.eirtrade.ie: No address associated with name

and

$ ping accounts.eirtrade.i
ping: cannot resolve accounts.eirtrade.i: Unknown host

The source of my interest is the next.

My sendmail receives mail from senders with domains like the first,
but rejects mail from the last:

------------------------------------------------------------------------
$ telnet localhost 25
Trying ::1...
telnet: connect to address ::1: Connection refused
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 host.my.domain ESMTP Sendmail 8.13.7/8.13.6; Mon, 4 Dec 2006 21:38:50 +0200
(EET)
HELO localhost
250 host.my.domain Hello localhost [127.0.0.1], pleased to meet you
MAIL From: asdf at accounts.eirtrade.ie
250 2.1.0 asdf at accounts.eirtrade.ie... Sender ok
RSET
250 2.0.0 Reset state
MAIL From: asdf at accounts.eirtrade.i
553 5.1.8 asdf at accounts.eirtrade.i... Domain of sender address 
asdf at accounts.eirtrade.i does not exist
QUIT
221 2.0.0 host.my.domain closing connection
Connection closed by foreign host.
------------------------------------------------------------------------

Elisej Babenko