You could receive several different error messages: “Connection Refused” or “Logging onto the Network Server Failed check username and password”. These are just two of the error messages that you could get.
You would be able to receive email but not send it.
Here is how to test it:
1. Go to 'Start' then 'Run' then type: cmd
2. When the Command prompt appears type: telnet mail.yourdomain.com 25
3. Wait a few seconds. You must see the following: '220 mail.yourdomain.com ESMTP'. This will be OK.
4. If you do not see this message, your SMTP is being blocked.
If you have a personal firewall, try lowering the security.
Call your ISP and ask if they block people sending through remote SMTP's. Some are doing that, though relatively few.
You can use your ISP's SMTP server. In your email client, put the SMTP server that you use to send out your ISP email, and your email will still appear with your domain name. Also see the specific error messages explanations.