Sending email messages

I live in England but often travel to France where I have an apartment. My
email provider in the UK is Onetel. In France I access the internet via Darty
Box. To send emails when I am in France I normally change the outgoing server
in windows mail to smtpauth.dbmail.com This has worked OK for some time. But
on my last two visits (April and August) I cannot send emails. The error
mesage is shown below:

The message could not be sent. The authentication setting might not be
correct for your outgoing e-mail [SMTP] server. For help solving this
problem, go to Help, search for "Troubleshoot Windows Mail", and read the
"I'm having problems sending e-mail" section. If you need help determining
the proper server settings, please contact your e-mail service provider.

The rejected e-mail address was ''. Subject 'Read:
Booking confirmation', Account: 'mail.onetel.com', Server:
'smtpauth.dbmail.com', Protocol: SMTP, Server Response: '554 5.7.1
<>: Recipient address rejected: Access denied', Port:
25, Secure(SSL): No, Server Error: 554, Error Number: 0x800CCC79