Blocking port 25 on your ISP for outgoing mail is one of the most common causes for inability to send emails using the mail client. In this case, you need to use port 26 instead of port 25 for outgoing mail in your mail client.
If you are using the Outlook mail client, please do the following to configure it to use port 26 :
1. Run Outlook and open the 'Tools' menu
2. Choose 'Accounts' or 'Account Options'
3. Open the 'Mail' tab
4. Click on 'Properties'
5. Click on the 'Advanced' tab
6. Change the outgoing mail from port 25 to port 26
7. Make sure the box that says "This connection requires a secure connection(SSL)" is NOT checked
8. Click OK
9. Restart Outlook