You can change which account is used to send a message:
1.Open Outlook
2.In the message window, above the To button, click From.
3.Click the account that you want to use.
Change your default email account:
1.You can change your default email account using the following steps.
2.Select File, then Account Settings, then Account Settings
3.From the list of accounts on the Email tab, select the account you want to use as the default account.
4.Select Set as Default, then Close
Always use the default email account for sending new email:
1.If you want to use the default email account when clicking New Email regardless what account or folder you start from use the following steps
2.Select File, then Options, then Mail
3.Under Send Messages, check the box Always use the default account when composing new messages
4.Select OK
Additional information can be found in Microsoft's knowledge base: https://support.microsoft.com/en-us/office/change-the-account-used-to-send-email-messages-2bdd8d4f-e30f-4ec8-88a0-406ce7b23cc5