Secondary email account

Set up a second user account on the computer for your wife.

Then setup Mail in her account with the default you wish her to use.

Otherwise one or the other of you will always have to select the From address for your email.

There can only be one default From address in Mail.

You have to set up another user account for your wife to use. You can't have two users on one account without both accounts being affected. Open Users & Groups preferences. Click on the lock icon and authenticate. Click on the Add [+] button to start a New Account. Select Standard from the dropdown menu. This will prevent your wife from being able to upgrade or update software you have downloaded nor to be able to make changes in or have access to your account. If you prefer she have equal access, then setup the account as an Administrator account. Now, click on Login Options button and click on the Show fast user switching menu as: checkbox, then select how you want it displayed in the menubar from the dropdown menu.

That's it. She now has an account of her own. If you set up a Standard account, then you can setup what she has access to, etc. using Screen Time preferences. See Use Screen Time on your Mac and Set up content and privacy restrictions in Screen Time on Mac.

You're welcome.

Select the account for Valerie. Click on the Delete [-] button, select the last option, Delete the home folder, then click on the Delete User button.

She may have to be logged out of her account before you can do that.