Hi,

According to my experience with Outlook 2010 , the problem is likely a restriction by your ISP (Internet Service Provider) as many block the normal sendmail port (25), due to their own security concerns. This can be the case even if sending was working recently as some ISP's roll these changes through without warning.

To test if port 25 is blocked, open your command line on your local computer. For windows users, you can access this via start -> run -> cmd.

If port 25 is blocked than you tried to fix this problem : If your ISP is blocking port 25, simply change your SMTP port number to 587. This setting is usually found in the advanced settings of your email client.