Server error 440, 441

I notice elsewhere that posters have been referred to BTInternet or B
Broadband.

I am currently getting this message with one single newsgroup:

""Outlook Express could not post your message. Subject 'Testing'
Account:
'news.btinternet.com', Server: 'news.btinternet.com', Protocol: NNTP
Server
Response: 'exim: failed to find uid for user name "mail"', Port: 119,
Secure(SSL): No, Error Number: 0x800CCCA9"


Another newsgroup poster has suggested that this is Server Erro
440,441.

Does anyone know what this is?

Nic

--
Nick