When sending mail with Outlook 2002, Outlook 2003 or Outlook 2007 there can be different reasons for receiving an error message containing the error code 0x8004210B. The whole message itself looks like this:
Task 'servername.com-Sending' reported error (0x8004210B): 'The operation timed out waiting for a response from the sending (SMTP) server. If you continue to receive this message contact your server administrator or Internet Service Provider (ISP).
Make sure all the settings for the account a configured correctly. One reason for this error message can be a wrong port number for the outgoing (SMTP) server. For example, you have set the port number to 465 (the port number for an encrypted connection), but you have forgotten to mark the SSL checkbox in the case of Outlook 2002 and 2007, or set the drop-down menu to SSL in Outlook 2007. In this case you'll get the error code 0x8004210B. If this is the cause for the problem, from the options for the email account just enable SSL encryption on port number 465, or, if you prefer to disable it, use the port number 587.
Another similar issue that can produce this error in Outlook 2007 and Outlook 2010 is when you choose TLS from the encryption drop-down menu for the outgoing (SMTP) server and you set the port number to 465. The encryption port number for SSL/TLS is actually 465, but what is meant in Outlook 2007 and 2010 by TLS is in fact STARTTLS. STARTTLS is not an encryption protocol but a command to start TLS. Unlike SSL and TLS, STARTTLS works on the non-encryption ports. So if you have selected TLS, set the port number for the outgoing server to 587, or alternatively leave the port number to 465 but change the encryption protocol from the drop-down menu to SSL.
Also make sure that the names for the incoming and outgoing mail servers are the correct ones and that they are the same.
Something else that can cause this error code is your antivirus software. Disable your antivirus application temporarily and test the email account again to see if the problem persists. If you can send emails, this means that obviously the reason is the antivirus application. With some antivirus applications you can disable the scanning of outgoing (and incoming) email messages. One antivirus system that is known to cause this problem is Norton Internet Security/Norton Antivirus.
If you need help with setting up a particular email client refer to the respective tutorial or article.