To have a Send-Only account, the is one from which mail will never be downloaded, you need to do the following:
1. In Outlook choose Tools, Options.
2. In Options go to Mail Setup, and click Send / Receive button.
3. Choose the account you want to edit and click Edit (Fig.1).
Fig.1. Mail Setup options
4. Point again to the account that you want to set Send-only property and clear Receive mail items field (Fig.2).
5. To approve click OK.
Fig.2. Send and Receive options.