By default, your email templates are pre-populated with a default greeting of Dear %firstname%. This is called a macro and it dynamically inserts the recipient's first name into the email at the time of delivery so your email will be addressed as Dear 'Bob' (where Bob is your supporter's first name).
When your email is sent to a record in your list who does not have a first name, the greeting reverts back to the default no-name greeting. Frequently, this a Dear 'Supporters' or Dear 'Friends' or Dear 'Friends and Neighbors'.