Windows Vista HP SP2 on Dell Inspiron 530S 2Gb 32-bit

Windows Mail 6.0

Full error message is

An unknown error has occurred. Subject 'xxxxxx', Account: 'xxxxxx', Server: 'mail.btinternet.com', Protocol: SMTP, Server Response: '421 Too many messages', Port: 25, Secure(SSL): No, Server Error: 421, Error Number: 0x800CCC67

Outgoing mail is SMTP to btinternet server (mail.btinternet.com)

Outgoing mail server has 'Requires Authentification' ticked (same settings as incoming).

If I re-try several times, eventually the mail will be sent OK.

We are only talking about 1 message per 'Send'.

I have recently deleted a load of messages from Inbox and Sendbox - but this has not helped.

I have tried changing port number - but get the same issue.

I don't know if the issue is with Windows Mail or with btinternet server.  The server is not storing an excessive number of messages.