Hi Mehnihma,
Before moving on, I want to confirm whether you have assigned the “Send as” permission. Please refer to the following article to check whether the Send as permission has been assigned.
http://help.outlook.com/en-us/140/ff852815.aspx
For Outlook 2011
1. You can refer to the following article to add a delegate of the email account that is used for sending as.
http://mac2.microsoft.com/help/office/14/en-us/outlook/item/567a9b64-1672-43e2-b751-31330352f5f5
2. After that, when composing a new email, the email address added as delegate will show under From in Outlook 2011.
For OWA
1. Click Options, select See all options
2. Click Settings
3. Select Always show From
4. After that, when composing a new email, you can choose the delegate mail address under From in OWA.
Thanks,
August Sun