[Zope-CMF] error joining a cmf site

[Roel Van den Bergh]

When trying to join our CMF site the new user get prompted with this error.
I just added some users last week without any problems and did not change
anything afterward (I hope nobody else).

Can some one help me out?
Zope 2.5.1 on linux with CMF 1.2

TIA Roel

Zope has encountered an error while publishing this resource.

Error Type: error
Error Value: host not found

Troubleshooting Suggestions

The URL may be incorrect.
The parameters passed to this resource may be incorrect.
A resource that this resource relies on may be encountering an error.
For more detailed information about the error, please refer to the HTML
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If the error persists please contact the site maintainer. Thank you for your
patience.

Traceback (innermost last):
  File /home/zope/2-5-0/lib/python/Products/Hotfix_2002-04-15/__init__.py,
line 51, in publish_module
  File /home/zope/2-5-0/lib/python/ZPublisher/Publish.py, line 114, in
publish
  File /home/zope/2-5-0/lib/python/Zope/__init__.py, line 159, in
zpublisher_exception_hook
    (Object: RMW)
  File /home/zope/2-5-0/lib/python/ZPublisher/Publish.py, line 98, in
publish
  File /home/zope/2-5-0/lib/python/ZPublisher/mapply.py, line 88, in mapply
    (Object: register)
  File /home/zope/2-5-0/lib/python/ZPublisher/Publish.py, line 39, in
call_object
    (Object: register)
  File /home/zope/2-5-0/lib/python/Shared/DC/Scripts/Bindings.py, line 252,
in __call__
    (Object: register)
  File /home/zope/2-5-0/lib/python/Shared/DC/Scripts/Bindings.py, line 283,
in _bindAndExec
    (Object: register)
  File /home/zope/2-5-0/lib/python/Products/CMFCore/FSPythonScript.py, line
115, in _exec
    (Object: register)
    (Info: ({'script': <FSPythonScript instance at 8ccd5c0>, 'context':
<CMFSite instance at 8baec18>, 'container': <CMFSite instance at 8baec18>,
'_': <TemplateDict object at 0xa6be178>, 'traverse_subpath': []},
('DeKeyserG', 'DeKeyserG'), {}, ('password', 'confirm')))
  File Script (Python), line 21, in register
  File /home/zope/2-5-0/lib/python/Products/CMFDefault/RegistrationTool.py,
line 127, in registeredNotify
    (Object: portal_registration)
  File /home/zope/2-5-0/lib/python/Products/MailHost/MailHost.py, line 156,
in send
    (Object: MailHost)
  File /home/zope/2-5-0/lib/python/Products/MailHost/MailHost.py, line 200,
in _send
    (Object: MailHost)
  File /home/zope/2-5-0/lib/python2.1/smtplib.py, line 187, in __init__
  File /home/zope/2-5-0/lib/python2.1/smtplib.py, line 222, in connect
error: (see above)