Unique Email Message Identififier?

What we’re looking for is a way to generate a unique identifier for each individual email that gets sent out by the email platform, or alternately, dynamic substitution strings or macros that change depending on when an email is opened by a user, in other platforms this would be similar to [date], or [time] macros. As an example, if I wanted the time to show up in an email that the user clicked on an email, using the [time] macro, it would show 2:22 PM. If I then opened the email again at 3:42, it would show 3:42 in the body of the email itself.

InfusionSoft support suggested we post here to see if anybody in the forum has found a workaround.

Thanks - Rodney