According to World Nutrition, the manufacturers of Vitalzym, Vitalzym Xe, and Vitalzym SEB, their systemic enzymes don’ t begin to lose activity until temperatures exceed 150 degrees.  Systemic enzymes are subjected to high temperatures during the manufacturing process so they must be able to resist these conditions.  So if your enzymes sit in a hot mailbox for an afternoon, they most likely have not been damaged.  Of course the first thing we do if we think something has gone bad is smell it.  Unfortunately this isn’t an indicator since systemic enzymes do have a sour smell to them even when they’re fresh.   If you’re concerned your order from www.ExpressNaturals.com has been subjected to extreme heat in excess of 150 degrees, send us an email at expresscare@expressnaturals.com.