Can't send email via smtp: authentication failure

Hello all,

I'm trying to send email with smtp, and I'm authenticating with the same username and password that I can use to successfully log in to webfaction's webmail, but when I try it with smtp it fails:

import smtplib
smtp = smtplib.SMTP()
smtp.connect('smtp.webfaction.com')
smtp.login('twneale@twneale.webfaction.com', '*****')

And I get this:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib/python2.7/smtplib.py", line 592, in login
  raise SMTPAuthenticationError(code, resp)
smtplib.SMTPAuthenticationError: (535, '5.7.0 Error: authentication failed: authentication failure')

I have also noticed that although I created an email address for "twneale@twneale.webfactional.com" in the control panel and sent up an autoresponder, it doesn't seem to be working. I also set up another email address with an autoresponder, and that one does work.

What am I doing wrong? Thanks!