Hello there, @sevenseasmc-ja.

QuickBooks Desktop (QBDT) makes it easy to change the email associated with your account as well as adding emails for your invoices. To do so, please refer to the steps below.

To change the email associated with your account:

1. Log in to the My Account site. 
2. From the Manage your QuickBooks page, select the product you would like to manage.
3. Click the drop-down arrow at the top right beside the company name.
4. Select Edit Login, then go to the Profile tab.
5. Click Edit in the Email address field.
6. Type your new email address in the Email address and Confirm email address field.
7. Enter your login password in the Password field.
8. Once done, click Save. This sends a verification email to your new email address.
9. Go to your email’s inbox and open the verification email. Click Verify my email.
10. Once you see the prompt Email address verified, select Continue to complete the email change.

For your reference, please see this article: Update the email address you use for QuickBooks Desktop.

Next, here are the steps you can perform in adding additional email in QBDT:

1. Navigate to the Edit menu and choose Preferences.
2. Click Send Forms and select My Preferences.
3. Choose WebMail.
4. Select Add.
5. Enter your email address on the Email Id field.
6. Then, pick the Email provider.
7. Once done, click OK.

When finished, simply click the drop-down arrow in the From field to select which email address you want to use in sending invoices. Check out this link for more details: Connect your email to QuickBooks Desktop.

In case you'll need to create additional email templates for your sales forms, you can head to this article for the detailed guide: Create custom email templates in QuickBooks Desktop.

Don't hesitate to post a reply below if you have additional questions in updating and adding emails in QBDT. I'm always here to help. Take good care.