Thank you for re-posting. My apologies, it seems it did get lost in a flurry of questions. You’re comfrey oil should not smell bad. I’m sorry to say that has gone off and will need to be discarded. Is this smell happening after it’s been strained, placed on a container and on the shelf for a bit, or right away as you were making the batch of oil? If it’s the first, it has likely gone rotten or fermented in an unpleasant way. If it is the latter, then it’s possible that you burned the oil?
The heat method with fresh plants, especially fresh plants that are as juicy as comfrey is not fool proof. In fact I don’t think I would recommend infusing fresh comfrey in oil at all, since it is such a watery/juicy plant. I would recommend drying it completely and using either warm or cold method or wilting it for 24-48 hours before doing the warm method.
Sorry to deliver the bad news and I hope you do another infusion with great success as comfrey oil is amazing to have on hand.