What does the following error message from Mailer-Daemon mean?
"Message from  yahoo.com.
Unable to deliver message to the following address(es).

<tflaherty@mchsi.com>:
204.127.203.150 failed after I sent the message.
Remote host said: 556 null byte in data"

The question is what does "556 null byte in data" mean as to why the
mail is returned?

This newsgroup post may be of interest:

http://groups.google.com/groups?selm=040b01c47e98%24c2370150%243a01280a%40phx.gbl&output=gplain

I use Eudora as my mail program. I did what the newsgroup suggested
and tried sending the message again. It said "The mail server does not
support SSL" so it wouldn't send my message. It didn't say which mail
server but since I am using SBC Yahoo I am assuming it was SBC Yahoo.

Dear sir, 

Try to send the e-mail directly from the web ( ex : if you are yahoo
subscriber, send from mail.yahoo.com ) . If you get the same response
, means that the email address you have sent to , does not exists.If
the email gets to the receiver ok , means that your mail client is not
configured properly.
Try reading Eudora's configuration manual 

Best regards,
Mihai

I tried to send directly from the Yahoo web site and it also comes
back for the same reason. The email address exists because the first
time I got it back was in reply to a message I had received from that
address. Previously we had  a lot of back and forth email activity
without a single message returned. This just started happening within
the last month. I haven't changed anything on my computer.