In e M client 7, how do I set a default e-mail address?

you can set a default eMail-Account, if that ́s what you need…

Check the Account-Settings. The Window shows a list of all your eMail-Accounts.
One of them shows a “star”-Symbol on the Envelope-Icon. That ́s the default-Account.

To change that, highlight the desired Account by clicking on it and the click on the Star-Button on the bottom of the Account-List.