By default, messages are sent based on the email account that you are viewing. For example, if you have added multiple email accounts, the mail folder that is open determines which email account is used.
Also, when you click Reply, Reply All, or Forward, your message is sent based on which account received the original message.
If multiple email accounts are configured in Microsoft Outlook 2013, then you can change which account is used to send a message.
In the message window, above to To button, click From.
Click the account that you want to use.
Why is the From button is missing?
If you see the Send button, but don’t see the From button, your Outlook profile contains only one email account.