A growing number of providers block the outgoing port (25). This can be resolved by setting the outgoing port to 2525.
The following solution will work in most cases:
In Outlook, click on “Account Settings” in the “Tools” menu. Go to the “Email” tab, click on the account you want to change and then click on “Change”.
In the “Change Email Account” dialogue, click on “More Settings”. Go to the “Server” tab for outgoing mail and check the “My outgoing server (SMTP) requires authentication” checkbox. Choose one of the following options: Click on “Use same settings as my incoming mail server” (!). This is the most commonly used option. Click on “Log on using” and enter your user name.
Note: Some providers (including KPN and Ziggo) block outgoing email servers. In that case, you will need to enter the server of your own provider. A growing number of providers block the outgoing port (25). This can be resolved by setting it to 2525, but that option is beyond our control.