problems sending email to various domains

[Lynn Smith]

We're having issues sending email to various domains - they are returned
with a message:
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.  
In fact, I'm unable to send to this list at isc.org.  I had to subscribe
to the list under a different email address to be able to post this.

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.  I'm assuming it's on my end, since I can go to dnsstuff.com and
don't see a problem when doing a lookup on the mx records.  But I'm
clueless as to how to fix the problem.