Hi Kelly,
The settings under Customize > My Account > Default Email Preference should be the default settings. You shouldn't have to go in there and switch it between you and Judy. You can contact your ISP to confirm what the settings there should be.
To specify the email address for each user, you need to go to Customize > Users and enter their email addresses in step 1 of 7. This way if you send an email from Method it would show that it was sent by you, and if Judy sends an email it will show that it was sent from her.
Hope this helps.