There are several ISPs who blocked the use of "Port 25" to limit on the amount of spam that is sent over their networks.
To check out if indeed your ISP has blocked the Port 25 (used by default for the SMTP server):
- Click on Start>Run>type cmd.
- In the command prompt use one of these command:
- telnet domain-name.com 25
- telnet smtp-server.domain.com 25
If it is the case, then you must configure Outlook to work with another SMTP server.
- Open Outlook>Tools> Options>Account settings.
- Double click to select a particular account.
- Click on the "More Settings" option
- In the Internet Email Settings window select the Advanced tab.
- From there simply change the port for the Outgoing server.