inability to send email - who's problem?

[Dana Holland]

We're having issues sending email to various domains - they are returned 
with a message:

Hi. This is the DeskNow mail server at mail.navarrocollege.edu.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up trying. Sorry it didn't work out.
These recipients did not receive the message:
ann at ipa.net

This is why I was unable to deliver your message:
Remote mail server at 207.69.189.21 replied: 550 Authentication 
required. Help at http://www.earthlink.net/go/emailsettings


We see these messages when we try to send to ipa.net, mindspring.com, 
and even earthlink.net.  We see them when trying to send to yahoogroups. 
  And we even seen them when trying to mail to various state.tx.us 
addresses.

A "dig ipa.net mx" returns:

; <<>> DiG 9.2.4 <<>> ipa.net mx
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 49491
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 13, ADDITIONAL: 0

;; QUESTION SECTION:
;ipa.net.                       IN      MX

;; AUTHORITY SECTION:
.                       518400  IN      NS      D.ROOT-SERVERS.net.
.                       518400  IN      NS      E.ROOT-SERVERS.net.
.                       518400  IN      NS      F.ROOT-SERVERS.net.
.                       518400  IN      NS      G.ROOT-SERVERS.net.
.                       518400  IN      NS      H.ROOT-SERVERS.net.
.                       518400  IN      NS      I.ROOT-SERVERS.net.
.                       518400  IN      NS      J.ROOT-SERVERS.net.
.                       518400  IN      NS      K.ROOT-SERVERS.net.
.                       518400  IN      NS      L.ROOT-SERVERS.net.
.                       518400  IN      NS      M.ROOT-SERVERS.net.
.                       518400  IN      NS      A.ROOT-SERVERS.net.
.                       518400  IN      NS      B.ROOT-SERVERS.net.
.                       518400  IN      NS      C.ROOT-SERVERS.net.

;; Query time: 3 msec
;; SERVER: 205.165.189.183#53(205.165.189.183)
;; WHEN: Mon Jan 29 08:14:36 2007
;; MSG SIZE  rcvd: 233

Is this a DNS problem on my end, or on the other end?  We're running 
bind 9.3.4.