Email Rules play a large part in how the From email address is populated when sending/replying. If you don't have any default rules set up, the From field will be blank.
You will need to set up at least a Default template and Editor Rule where you can specify a From email address.
If you reply to an email which is not related to an object in Jim2, it will pick up the Template rule that you have set up. For example, if an email is linked to a purchase, the accounts@happen.biz email address will appear in the From field.
Further information: