Once the 'modal' window goes away, now you're looking at the Preferences/Accounts window again.Tick the next two checkboxes and enter the number 465 to the right of where you entered Hitting Tab actually makes the window refresh and now you have more fields for entering data. In the new window that pops up, enter for the email address, then hit Tab to go to the next field.Click and hold the + sign in the lower left, and select Email.Make sure only your Exchange account is listed there. In Outlook (for Mac), go to Preferences > Accounts.As an added benefit, this will still allow you to use authenticated SMTP to send these messages, preventing them from being flagged as spam and maintaining the ability to verify the sender of the message. This second account can be configured alongside the personal Exchange account.
The general solution is to set up a separate email account in your mail program specifically for sending mail with this 'fake' FROM address via MIT's SMTP server, rather than via the Exchange environment.