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.