[Zope] Error with MailHost object

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@bbs-one.fr': (550, 'RCPT TO:<sebastientessier@bbs-one.fr> Relaying not allowed')} I use Zope 2.6.2 with windows 2000. If someone can explain this to me...