Outlook Express not sending messages

[Jonathan Robert Muirhead]

Dear folks,

Every time I try to send a message using Outlook Express,
it gives me the following message:

The message could not be sent because one of the
recipients was rejected by the server. The rejected e-
mail address was '(e-mail address removed)'.
Subject '', Account: 'in.talk21.com',
Server: 'out.talk21.com', Protocol: SMTP, Server
Response: '550 relaying mail to comedyunit.co.uk is not
allowed', Port: 25, Secure(SSL): No, Server Error: 550,
Error Number: 0x800CCC79

What can I do to rectify this?

 

[Vanguard]

Is talk21.com *your* Internet provider? If not, they are not allowing
you to relay SMTP traffic across their network to reach an SMTP server
off their network. That's what spammers try to do to abuse a network
and to hide themselves. If talk21.com is your ISP then maybe you need
to authenticate to their server (by specifying the username and password
to the SMTP server, or maybe just reuse the ones specified for the POP3
server).