I'd like to see the option for different external tracking strings by email group. Today, we have the option for a single external tracking string -- and it's either on for all emails or off for all emails. Offering this per email group would enable us to use strings for some emails and not for others, but even more, it would let us supply different strings for different email typess. Depending on the way companies use email groups, this would let there be unique tracking by marketing campaign, or preference center selections, or audience types and email type (promotion, newsletter, form confirmation, monetary transaction, etc.).