#157 Not able to send emails

[canditar]

Any time I try to send an email I get the following error msg: Server Response: '554 Mailbox unavailable.'. (Account: '127.0.0.1', SMTP Server: '127.0.0.1', Error Number: 0x800ccc6f. Later when I tried to send again I got an error msg from YPOPs! that sd: Yahoo reported "Mising credentials" error - try lowering your cookie timeout in half ("Keep session alive" under Session) and restart ypops.

I am completely confused & have looked all over the place trying to find a fix for this. I'm running Vista, if that helps. Any help would be greatly appreciated, as I'm getting very frustrated.
Thanks,
Candi