"From: address is not valid." when sending mail

[levander]

I'm trying to set up smtpmail to send mail from gnus and the defaul
mail user agent.  I've got most of it working.  But, there's some
error when smtpmail is sending the email to my ISP's mail server.
>From the smtp trace in emacs, note that highhat.net is a private
domain name that I just made up here, mindspring.com is the mail
domain, earthlink.net is the domain that hosts the mail server for
mindspring:

Process SMTP deleted
220-elasmtp-curtail.atl.sa.earthlink.net ESMTP Exim 4.34 #1 Fri, 29
Jun 2007 18:10:31 -0400
220-NO UCE.  EarthLink does not authorize the use of its computers or
network
220 equipment to accept, transmit, or distribute unsolicited e-mail.
EHLO louis.highhat.net
250-elasmtp-curtail.atl.sa.earthlink.net Hello louis.highhat.net
[66.32.215.156]
250-SIZE 14680064
250-PIPELINING
250-AUTH PLAIN LOGIN CRAM-MD5
250-STARTTLS
250 HELP
AUTH PLAIN [was some long encoded looking string - removed cause I
didn't know if it was sensitive info]
235 Authentication succeeded
MAIL FROM:<levander@highhat.net> SIZE=18868
250 OK
RCPT TO:<levander@mindspring.com>
550 From: address is not valid.   Please check your email settings.
QUIT
221 elasmtp-curtail.atl.sa.earthlink.net closing connection



Is that "MAIL FROM:" being sent to the server, is that the same as the
"From" header in the composed email itself.  If so, I don't know why
it's being set to highhat.net, as I'm using styles in gnus to set the
from address to "levander@mindspring.com", and I see that header field
in the window when I'm composing the email.

Maybe that "MAIL FROM:" being sent to the server is coming from /etc/
mailname?  I do have "highat.net" in that file.  But, I'd rather not
change the contents of that file.  Is there another way to specify the
domain sent to the server in that "MAIL FROM:" string?

My smtp-auth-credentials does have "levander@mindspring.com" set as
the username.  If it wasn't, I don't think authentication would
succeed in that trace.