You can use an S1 tag to pull in the contact record's casual salutation for emails:

[[S1:salutation_casual]]

However, you have to make sure every record has that field or else you miss an opportunity to make the intro personal. Here's a conditional string that substitutes "Friend" if both salutation fields and the first name field is blank: 

[[?xx[[S1:salutation_formal]]xx::xxxx::[[?xx[[S1:salutation_casual]]xx::xxxx::[[?xx[[S1:first_name]]xx::xxxx::Friend::[[S1:first_name]]]]::[[S1:salutation_casual]]]]::[[S1:salutation_formal]]]]