Wait, a wrong DNS record took 3 weeks to track down? That seems off to me. A single bad record usually shows up fast if you know where to look, like 15 minutes of digging and you'd spot the problem. The only way it drags on that long is if you're chasing it blind without checking the actual logs or zone files first. I get that some email stuff gets weird with caching and propagation, but 3 weeks sounds more like there were several things broken and not just one record. Not trying to call you out, but the story doesn't add up the way you told it.