[Zope] Error with MailHost object

[Sébastien Tessier]

Hi,

I am trying to send mails with the mailhost object using the 'send' 
method and, according to whether the SMTP server I use, sometimes it 
works and sometimes I have the following error : SMTPRecipientsRefused: 
{'sebastientessier at bbs-one.fr': (550, 'RCPT 
TO:<sebastientessier at bbs-one.fr> Relaying not allowed')}

I use Zope 2.6.2 with windows 2000.
If someone can explain this to me...