Hi
I too am having problems sending email from my Yahoo accounts. I contacted Yahoo support and they sent me this to try.
1. Launch the "Terminal" application located in the "Applications:
Utilities" folder.
2. At the text prompt, type "telnet pop.mail.yahoo.co.uk 110" and press
the "Enter/Return" key. If you're successful, you should now see "+OK
hello from popgate." If you don't see this, close the window and start
over or check your Internet or network connection to make sure you're
logged on.
3. From this screen, type "user xxx" (where xxx is your Yahoo! ID) and
press "Enter/Return." You should be prompted with "+OK password
required."
4. Type "pass yyy" (where yyy is your case-sensitive password) and press
"Enter/Return." If this is successful, you should receive a prompt with
"+OK maildrop ready," followed by mailbox details. If this was not
successful, you'll receive "-ERR Error logging in. Please visit
http://ymailuk.com/."
5. Type "quit" and press "Enter" to sign off the POP server.
6. Type "exit" and press "Enter" to return to Windows.
If you weren't able to get the POP authentication during the first try,
please wait a few minutes and try again. After several tries, you should
be able to tell if your ID and password will allow you to achieve POP
authentication.
Next, check if you are able to communicate with the Yahoo! Mail Plus
SMTP server. Here's how:
1. Launch the "Terminal" application located in the "OS X Applications:
Utilities" folder.
2. At the text prompt, type "telnet smtp.mail.yahoo.co.uk 25" and press
the "Enter/Return" key. If the port is not blocked, you should receive a
response similar to "220 smtpxxx.mail.yahoo.co.uk ESMTP" (where "xxx" is
the server number you connected to). If you don't receive this specific
response or if you get a different response, your ISP, networking
equipment, or security software may be blocking or scanning outgoing
mail servers.
NOTE: Please also try port 587 in place of 25. It's important that you
try the alternate port, as some ISPs will block port 25 by default.
Though it does mention Mail Plus in the instructions and I don't have MailPlus I was able to connect using port 587 to the SMTP Server.