There could be a couple of reasons for this:
1. Yahoo have blocked your sending emails as you have exceeded their limit, but this would normally be reset after a time period. I tried Yahoo a long time ago and found it didnt work well with PDW. Try using GMail I have had a lot of success with GMail but they too block emails after a certain amount, form memory based on the number recieved per hour.
2. You may have been blacklisted. Be very careful where you send your emails with PDW. If you send emails to dodgy email addresses that get bounced you could end up on some internet blacklists which will make sending very difficult.
3. Your ISP might be blocking port 25 to limit the amount of email traffic through their servers.
4. Lastly confirm your SMTP details are correct. I know I have been guilty of inputing the details before
If you are still having trouble try uising a mobile broadband device to see if you can get your emails out, this may help to identify whether your problem is ISP related or not.
Cheers!
