In many cases, ISP's have started blocking traffic on outgoing mail (port 25). You can change to our alternate port, which should resolve this issue
1. In Outlook, drop down the Tools menu.2. Choose Accounts or Account Options.
3. Click on 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 that the box that says "This connection requires a secure connection(SSL)" is NOT checked.
8. Click OK.
9. Restart Outlook.