There's no way to know for certain what's in the flavoring, how it was processed, or what source material was used for the flavoring. It might be highly toxic, perfectly harmless, or anything in between.
It's up to you if you want to enjoy this one box you were gifted- it depends on how sensitive you are to additives. I allow my children to enjoy "less than perfectly healthy" things their friends have given them (including a carageenan-laden chocolate milk mix given as a hostess gift) but I'm much more careful personally because I'm so sensitive and my health is more fragile.
The amount of toxins found in flavored tea bags is overall pretty small; the whole box is probably less toxic than a single bag of potato chips or one serving of processed cake or cookies. If you're basically healthy, it's probably not worth worrying about. Enjoy the box and then buy something healthier when it's used up.