If you have multiple accounts in Outlook, when you compose a new email message, Outlook picks the default account as the From account. However, if you want to Reply, Reply All, or Forward a message, Outlook will send your message with the same account the original mail was sent to. For example, Cheryl has two accounts. Her default account is firstname.lastname@example.org. Her second account is email@example.com. If Cheryl receives an email message from Chester at her firstname.lastname@example.org account, when she replies to Chester's message, her reply will be sent using email@example.com. If she creates a new message to Chester that's not a reply or a forward, the message will be sent using her firstname.lastname@example.org account.
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, the typical cause is that your Outlook profile contains only one email account. To view the From button, you'll need to add another email account.
If you have multiple email accounts, you can add the From button manually. Once you add the From button, you can't remove it unless you remove all but one email account from Outlook.
Create a new email message.
Select Options, then select From in the Show Fields group on the ribbon.
Change your default email account
You can change your default email account using the following steps.
Select File > Account Settings > Account Settings.
From the list of accounts on the Email tab, select the account you want to use as the default account.
Select Set as Default > Close.