Cannot send e-mail from Outlook Express 6 - get error 'Server Error: 250, Error Number: 0x800CCC7D'

I can receive e-mail in Outlook Express 6 ok, but when I try to send e-mail I get the following error message - 'The server does not support a SSL connection. Account: 'pop3.live.com', Server: 'smtp.live.com', Protocol: SMTP, Server Response: '250 OK', Port: 25, Secure(SSL): Yes, Server Error: 250, Error Number: 0x800CCC7D'.

If I go into Accounts/Properties/Advanced and uncheck the 'This server requires a secure connection (SSL)' box for 'Outgoing mail (SMTP)', I get the error message - 'The message could not be sent because the server rejected the sender's e-mail address. The sender's e-mail address was '*** Email address is removed for privacy ***'. Subject 'Test message', Account: 'pop3.live.com', Server: 'smtp.live.com', Protocol: SMTP, Server Response: '530 5.7.0 Must issue a STARTTLS command first', Port: 25, Secure(SSL): No, Server Error: 530, Error Number: 0x800CCC78'.

The only thing I have changed on my PC recently is IE - upgraded to IE8.
I don't know if that is related to this problem. I only use Outlook Express 6 occasionally for sending e-mail, so I don't know if this problem started when I upgraded to IE8 or not. My ISP is MSN.