I have look at some of the threads with the same problem and there does not seem to be a solution to this issue. One minute it was working fine, receiving and sending emails and the next it would not send email at all. It kept asking for my username and password indicating that my ISP required these log in details. I logged onto my ISP using webmail function(mail.btinternet.com) and I was able to receive and send email without a problem. So, the problem is with Windows Mail. I then checked my account and noticed that my pop3 and smpt details had been deleted 'mysteriously'. I set up another account with my details again and then tried to receive/send. It receives okay, but will not send and because it keeps asking for my username and password. When I enter these details, it either times out trying to connect or gives an error requesting the log in details again. This is frustrating and not how it is supposed to work. I prefer XP to this crap !!
I get a security box asking for my username and password to my ISP
I don't really know if it's common, but I have only recently switched to BT, and my other provider used @ not. and it's an easy mistake to make. If I were you I would delete all accounts in MS Mail and then re-enter them. Perhaps delete them,reboot and re enter.
smtp.mail.yahoo.com as your smtp server setting. You might find it works.
The other thing to check is that you have entered the correct setting for the smtp server authentication. The smtp server will require you to authenticate yourself with your BT email address and username.
Click on Tools, Accounts and highlight the email account in the Internet Accounts window. Click on Properties and then click on the Advanced tab. Change the outgoing SMTP port from 25 to 587. I had the same problem using Outlook 2003 and Vista and found this instruction in the BT Yahoo Network Status page. Curiously it is no longer there. Let me know if this solution works.
I am having the same problem with Vista Mail and my ISP Tiscali. Brand new PC, set-up fine and was all working. Now Vista mail stopped sending emails they are all stacking up in my Outbox but I am receiving OK. Checked Pop & Smtp settings which are OK and not been changed. Ticked the box saying my Server Requires Authentication - didn't make any difference (as I know BT ISP needs this box ticked).